Free ASCP MLS Exam Practice Questions Mock Test: Part 27 – Microbiology – Fungal Identification & Infections

Coccidioides immitis (and Coccidioides posadasii) is a dimorphic fungus. In its tissue phase (within the human host at 37°C), it forms characteristic spherules—large, thick-walled, spherical structures that contain numerous small, internal endospores. When the spherule ruptures, it releases these endospores, which can disseminate and develop into new spherules. This structure is pathognomonic for coccidioidomycosis (Valley fever).

Why not the others?

• b) Yeast with broad-based buds: Typical of Blastomyces dermatitidis in tissue.

• c) Pseudohyphae: Seen in Candida species.

• d) Septate hyphae with conidiophores: Found in the environmental (mold) form of Aspergillus and other fungi, not the tissue form of Coccidioides.

Sabouraud dextrose agar (SDA) supplemented with chloramphenicol and cycloheximide is the standard medium used to isolate pathogenic fungi while suppressing saprophytic (environmental) molds and bacteria:

• Chloramphenicol: Inhibits bacterial growth.

• Cycloheximide: Suppresses saprophytic fungi (non-pathogenic molds) that may overgrow slower-growing pathogenic fungi.

This selective medium is particularly useful for isolating dermatophytes, dimorphic fungi (e.g., Histoplasma, Blastomyces), and other pathogens from contaminated clinical specimens.

Why not the others?

• a) Blood agar: Supports the growth of both bacteria and fungi but lacks selective inhibitors for saprophytic molds.

• b) MacConkey agar: Selective for Gram-negative bacteria and inhibits fungi and Gram-positive bacteria.

• d) Chocolate agar: Enriched for fastidious bacteria (e.g., Haemophilus) but does not suppress saprophytic molds.

The presence of non-septate (or sparsely septate), ribbon-like hyphae with right-angle (90°) branching is highly characteristic of fungi in the Zygomycetes class (e.g., Rhizopus, Mucor, Lichtheimia). These molds cause mucormycosis and are angioinvasive, leading to tissue necrosis. This morphology is a key diagnostic feature in tissue samples (e.g., KOH prep or histopathology).

Why not the others?

• a) Aspergillus: Produces septate hyphae with acute-angle (45°) branching.

• c) Dermatophytes: Have septate hyphae but do not exhibit right-angle branching in tissue.

• d) Fusarium: Produces septate hyphae that may branch at acute angles, similar to Aspergillus.

The finding of septate hyphae and arthroconidia in a KOH preparation of skin scrapings is highly characteristic of a dermatophyte infection (e.g., tinea corporis, tinea pedis, tinea cruris).

• Septate hyphae: Long, branching filaments with cross-walls, typical of dermatophytes.

• Arthroconidia: Barrel-shaped asexual spores formed by the fragmentation of hyphae, often seen in dermatophyte infections.

Dermatophytes (e.g., Trichophyton, Microsporum, Epidermophyton) infect keratinized tissues like skin, hair, and nails, and this microscopic appearance is classic for such infections.

Why not the others?

• a) Candida infection: Typically shows budding yeast cells and pseudohyphae (elongated filamentous forms with constrictions at septa), not true hyphae with arthroconidia.

• c) Mucormycosis: Caused by zygomycetes (e.g., Mucor, Rhizopus), which produce broad, nonseptate (or sparsely septate) hyphae with right-angle branching—not septate hyphae with arthroconidia.

• d) Pseudallescheriasis: Caused by Pseudallescheria boydii (asexual form: Scedosporium apiospermum), which produces septate hyphae but typically not arthroconidia in tissue; it may resemble Aspergillus.

• Calcofluor white is a fluorescent dye that binds to chitin and cellulose in fungal cell walls.

• When viewed under an ultraviolet (UV) or fluorescent microscope, the fungus will fluoresce a bright apple-green or blue-white color, making it easily visible even in low concentrations or against a complex background. It is often used in conjunction with KOH (option b) to clear the specimen, but the defining characteristic is its fluorescent property targeting chitin.

The other options are incorrect:

• a) Gram stain: This is a bacterial stain that classifies bacteria based on cell wall properties (Gram-positive vs. Gram-negative). Fungi are generally Gram-positive but this is not a specific or optimal stain for them.

• b) KOH preparation: Potassium hydroxide (KOH) is used to dissolve keratin and human cells in a specimen to make fungal elements more visible under a light microscope. However, it is not a stain itself; it is a clearing agent. Sometimes calcofluor white is added to a KOH preparation to enhance detection.

• d) Acid-fast stain: This is used primarily to identify Mycobacterium species (e.g., M. tuberculosis), which retain dye after acid washing. It is not used for fungi.

• Mycetoma is a chronic subcutaneous infection characterized by swollen lesions and granules (grains) that are discharged from sinus tracts. It can be caused by fungi (eumycetoma) or bacteria (actinomycetoma).

• Pseudallescheria boydii (and its asexual form, Scedosporium apiospermum) is one of the most common fungal causes of eumycetoma, especially in temperate regions.

The other options are incorrect:

• b) Candida albicans: Causes opportunistic infections like thrush and vaginitis, but not mycetoma.

• c) Cryptococcus neoformans: Causes cryptococcosis, primarily meningitis, and is not associated with mycetoma.

• d) Malassezia furfur: Causes superficial skin infections like tinea versicolor and folliculitis, but not mycetoma.

Tinea nigra is a superficial fungal infection characterized by brown to black pigmented macules on the palms (or less commonly, the soles). It is caused by Hortaea werneckii (formerly known as Exophiala werneckii or Cladosporium werneckii), a dematiaceous (dark-pigmented) fungus. The infection is asymptomatic and often acquired from soil, wood, or decaying organic matter in tropical and subtropical regions.

Why not the others?

• a) Trichophyton rubrum: Causes dermatophytoses like athlete’s foot (tinea pedis) or ringworm (tinea corporis), but not tinea nigra.

• c) Malassezia furfur: Causes tinea versicolor (pityriasis versicolor), which presents as hypopigmented or hyperpigmented patches on the trunk, not palmar macules.

• d) Candida albicans: Causes candidiasis (e.g., thrush, vaginitis) and may involve skin folds but does not cause tinea nigra.

The presence of oval budding yeasts in superficial infections of the mouth (e.g., thrush) or vagina (e.g., vulvovaginitis) is classic for candidiasis, most commonly caused by Candida albicans. These yeasts may also form pseudohyphae (elongated filamentous cells) in tissues, which helps differentiate them from other fungi.

Why not the others?

• a) Dermatophytosis: Caused by dermatophytes (e.g., Trichophyton), which appear as septate hyphae and arthroconidia in KOH preparations, not budding yeasts.

• c) Aspergillosis: Caused by Aspergillus species, which are molds producing septate hyphae with acute-angle branching in tissues, not yeasts.

• d) Cryptococcosis: Caused by Cryptococcus neoformans, which is a encapsulated yeast but typically causes systemic infections (e.g., meningitis) rather than superficial oral/vaginal infections.

Candida glabrata is characterized by its unique morphology compared to other Candida species:

• It grows as small, creamy, white colonies on agar plates (resembling Candida albicans but typically more modest in size).

• It exists primarily as small, budding yeast cells and does not form pseudohyphae or true hyphae under standard conditions. This absence of hyphal forms is a key distinguishing feature.

Why not the others?

• a) Formation of germ tubes: This is characteristic of Candida albicans (and C. dubliniensis), not C. glabrata.

• b) Production of pseudohyphae and true hyphae: Seen in species like C. albicans, C. tropicalis, and C. krusei, but not in C. glabrata.

• d) Production of a capsule: Typical of Cryptococcus neoformans, not any Candida species.

The India ink preparation is a rapid, classic test used to visualize the thick polysaccharide capsule of Cryptococcus neoformans in clinical specimens, particularly cerebrospinal fluid (CSF). The capsule excludes the ink particles, creating a clear halo around the yeast cell against the dark background. This is a key diagnostic feature for cryptococcal meningitis, especially in immunocompromised patients (e.g., those with AIDS).

Why not the others?

• a) Candida pseudohyphae: Identified via KOH prep or Gram stain, not India ink.

• c) Aspergillus conidia: Observed using lactophenol cotton blue from culture, not India ink.

• d) Dermatophyte hyphae: Detected by KOH prep of skin/hair/nail samples.

“Rainwheel” (also known as “racquet”) hyphae are specialized structures often observed in the microscopic morphology of dermatophytes (e.g., Trichophyton, Microsporum, Epidermophyton) when cultured on agar. These hyphae resemble a series of stacked cups or a racquet shape, formed by consecutive swollen segments. They are not pathogenic structures but are useful clues for identifying dermatophytes in laboratory cultures.

Why not the others?

• a) Aspergillus fumigatus: Identified by septate hyphae with acute-angle branching and conidiophores ending in vesicles, not racquet hyphae.

• c) Zygomycetes (e.g., Mucor, Rhizopus): Characterized by broad, nonseptate hyphae with right-angle branching, and structures like sporangiophores and rhizoids.

• d) Candida species: Typically show budding yeast, pseudohyphae, or true hyphae (e.g., C. albicans), but not racquet hyphae.

• Blastomyces dermatitidis is a dimorphic fungus. In the human body (at 37°C), it grows as a yeast characterized by large, spherical cells that produce single buds with a characteristically broad base (the bud is nearly as wide as the parent cell). This is a key diagnostic feature.

• Contrast with the other options:

  • a) Histoplasma capsulatum: At 37°C, it appears as small, oval yeasts often found within macrophages. It buds with a narrow base.

  • c) Coccidioides immitis: This is also dimorphic, but at 37°C it does not form buds. Instead, it forms spherules that contain endospores.

  • d) Sporothrix schenckii: At 37°C, it appears as cigar-shaped yeasts that can bud, but the buds are not characteristically broad-based.

The definitive diagnostic feature of Coccidioides immitis (and Coccidioides posadasii) in tissue is the presence of spherules (large, thick-walled, spherical structures) filled with endospores. When the spherule ruptures, it releases these endospores, which can disseminate and develop into new spherules. This structure is pathognomonic for coccidioidomycosis (Valley fever) and is a key differentiator from other fungal infections.

Why not the others?

• a) Broad-based budding yeast: Characteristic of Blastomyces dermatitidis in tissue.

• b) Small intracellular yeast: Typical of Histoplasma capsulatum, which is often found within macrophages.

• d) Septate hyphae with 45-degree angle branching: Seen in Aspergillus species and other hyaline molds, not Coccidioides in its tissue phase.

The KOH (potassium hydroxide) mount is a simple and rapid technique used to visualize fungal hyphae, spores, or yeast cells in skin, hair, or nail specimens. KOH dissolves keratinized debris (e.g., skin cells, nail material) by breaking down proteins and lipids, clearing the background and making fungal elements more visible under a microscope. This method is particularly useful for diagnosing superficial fungal infections like dermatophytosis, candidiasis, or tinea versicolor.

Why not the others?

• a) Gram staining: Primarily used for bacteria; fungi may stain but keratin debris is not dissolved, leading to poor visualization.

• c) Ziehl-Neelsen staining: Used for acid-fast bacteria (e.g., Mycobacterium tuberculosis), not fungi.

• d) Lactophenol cotton blue: Used for staining fungi from culture (e.g., to observe conidia or hyphal structures), but it does not dissolve keratin and is not suitable for direct clinical specimens like skin scrapings.

Cryptococcal meningitis, caused by Cryptococcus neoformans (and less commonly C. gattii), is one of the most common life-threatening fungal infections in AIDS patients (especially those with CD4 counts <100 cells/μL). The fungus is inhaled from the environment (e.g., soil, bird droppings) and disseminates to the central nervous system, leading to meningitis. Symptoms include headache, fever, nausea, and altered mental status.

Why not the others?

• a) Histoplasma: Histoplasma capsulatum can cause disseminated disease and meningitis in AIDS patients but is less common than cryptococcosis.

• c) Blastomyces: Blastomyces dermatitidis rarely causes meningitis and is not strongly associated with AIDS.

• d) Sporothrix schenckii: Causes sporotrichosis (skin/nodular infections) and is not a typical cause of meningitis in immunocompromised hosts.

Histoplasmosis, caused by the dimorphic fungus Histoplasma capsulatum, often resembles tuberculosis histologically due to its ability to cause:

• Granulomatous inflammation: Characterized by organized collections of macrophages (granulomas), giant cells, and lymphocytes, similar to tuberculosis.

• Necrosis: Caseating granulomas may form, mirroring the pathology seen in TB.

• Chronic pulmonary disease: Symptoms and radiographic findings (e.g., cavitary lesions, hilar lymphadenopathy) can mimic TB.

This similarity is why histoplasmosis is sometimes called “Darling’s disease” and must be differentiated from TB in endemic areas (e.g., Ohio and Mississippi River valleys).

Why not the others?

• a) Candidiasis: Causes suppurative (pus-forming) infections or thrush, not granulomatous reactions.

• b) Cryptococcosis: Typically produces gelatinous capsules and lacks well-formed granulomas in immunocompromised hosts (though granulomas can occur in immunocompetent individuals).

• d) Tinea versicolor: A superficial skin infection with no systemic or granulomatous features.

• Microsporum audouinii is a classic cause of tinea capitis (scalp ringworm). When this dermatophyte infects hair, it produces spores on the outside of the hair shaft (ectothrix infection), which fluoresce a distinctive apple-green or yellow-green color under an ultraviolet Wood’s lamp.

• The other options are incorrect:

  • a) Trichophyton rubrum: A very common cause of various tinea infections (e.g., corporis, pedis), but it does not fluoresce under a Wood’s lamp.

  • c) Epidermophyton floccosum: A cause of tinea corporis, cruris, and pedis, but it does not infect hair and therefore does not fluoresce.

  • d) Candida albicans: This is a yeast that does not fluoresce under a Wood’s lamp and is not a common cause of tinea capitis.

Calcofluor white is a fluorescent dye that binds specifically to chitin (a key component of fungal cell walls) and cellulose. When viewed under an ultraviolet (UV) or fluorescent microscope, fungal elements (hyphae, spores, yeast cells) fluoresce a bright apple-green or blue-white color. This stain is often used in combination with KOH to enhance the detection of fungi in clinical specimens (e.g., skin, hair, or respiratory samples).

Why not the others?

• a) Gram stain: Primarily stains bacterial cell walls (peptidoglycan) and may weakly stain fungi but does not target chitin specifically or produce fluorescence.

• b) KOH: Dissolves keratin and debris to clear specimens for microscopy but is not a stain and does not fluoresce.

• d) Acid-fast stain: Targets mycolic acid in the cell walls of Mycobacterium species (e.g., tuberculosis), not chitin.

Pneumocystis jirovecii (formerly P. carinii), the causative agent of pneumocystis pneumonia (PCP), is best detected using the Gomori methenamine silver (GMS) stain. This stain specifically highlights the fungal cell wall, turning the cysts black or brown against a green background. The characteristic appearance is of collapsed cup-shaped cysts or helmet-shaped structures, which are diagnostic for PCP.

Why not the others?

• a) Gram stain: P. jirovecii stains poorly with Gram stain and is not reliably visualized; it may appear as faint, irregular outlines but is not definitive.

• b) KOH preparation: Used for clearing skin and nail specimens for dermatophytes but does not stain P. jirovecii effectively.

• d) Lactophenol cotton blue: Used for microscopic examination of molds and yeasts from culture, but it is not optimal for clinical specimens like bronchoalveolar lavage (BAL) fluid for PCP diagnosis.

Fluconazole is an azole antifungal agent that works by inhibiting ergosterol synthesis in the fungal cell membrane. Specifically, it targets the enzyme lanosterol 14-α-demethylase, which is crucial for converting lanosterol to ergosterol. This disruption leads to accumulation of toxic sterol intermediates and compromises membrane integrity, ultimately killing the fungus.

Why not the others?

• a) Amphotericin B: Binds directly to ergosterol in the membrane, forming pores that cause leakage of cellular contents (does not inhibit synthesis).

• b) Flucytosine: Converted to 5-fluorouracil inside fungal cells, inhibiting RNA and DNA synthesis (not ergosterol synthesis).

• c) Caspofungin: Inhibits the synthesis of β-(1,3)-D-glucan, a key component of the fungal cell wall (not the membrane).

Ergosterol is a sterol compound that is a key component of fungal cell membranes, serving a role similar to cholesterol in mammalian cell membranes. It is not found in:

• a) Plant cells: These use phytosterols (e.g., stigmasterol).

• b) Mammalian cells: These use cholesterol.

• d) Bacterial membranes: Bacteria lack sterols entirely (except for rare exceptions like Mycoplasma).

The phenol oxidase test (using media like birdseed agar or caffeic acid agar) is the most specific test for differentiating Cryptococcus neoformans from other nonpigmented yeasts. This test detects the enzyme phenol oxidase, which C. neoformans produces. When grown on these media, the yeast forms melanin, resulting in brown to black pigmentation of the colonies. This is a unique characteristic not shared by most other nonpigmented yeasts (e.g., Candida species).

Why not the others?

• a) Germ tube test: Used to identify Candida albicans (which produces germ tubes), but Cryptococcus is negative and does not form germ tubes.

• b) Urease test: Cryptococcus is urease-positive, but other yeasts like Rhodotorula and some Trichosporon species can also be urease-positive, so it is not definitive.

• d) Carbohydrate assimilation: Useful for speciation of yeasts but is slower and less specific than the phenol oxidase test for rapid identification of C. neoformans.

The satellite phenomenon refers to the growth of small colonies of Haemophilus influenzae around colonies of Staphylococcus aureus on blood agar. This occurs because:

• Haemophilus influenzae requires both X factor (hemin) and V factor (NAD) for growth.

• Staphylococcus aureus produces and releases V factor (NAD) into the surrounding medium as it grows.

• H. influenzae can then utilize this V factor, allowing it to form colonies only in the vicinity of S. aureus (the “satellite” colonies).

This test is used to identify H. influenzae and differentiate it from other bacteria.

Why not the others?

• b) Candida albicans: Does not exhibit satelliting; it is identified by germ tube tests or chromogenic agar.

• c) Cryptococcus neoformans: Grows independently and is identified by capsule production (India ink), urease test, and phenol oxidase test.

• d) Dermatophytes: Grow as molds without specific satelliting requirements; identified by macroconidia/microconidia.

• Otomycosis is a fungal infection of the external ear canal. Aspergillus species, particularly Aspergillus fumigatus and Aspergillus niger, are the most common causes.

Talaromyces marneffei (formerly Penicillium marneffei) is an opportunistic dimorphic fungus that primarily infects immunocompromised individuals, particularly those with advanced HIV/AIDS (e.g., CD4 counts <100 cells/μL). It is endemic in Southeast Asia and southern China. Infections often present as disseminated disease with fever, skin lesions, lymphadenopathy, and organ involvement.

Why not the others?

• a) Diabetics: While diabetics are at risk for other fungal infections (e.g., mucormycosis), T. marneffei is not specifically associated with diabetes.

• c) Immunocompetent individuals: Rarely affected; infection is overwhelmingly linked to immunosuppression.

• d) Neonates: Not a typical risk group; cases are exceptionally rare and usually associated with underlying immune defects.

The combination of a dematiaceous mold (dark-pigmented fungus due to melanin in the cell wall) and sclerotic bodies (also called muriform bodies or copper-penny bodies) in a skin scraping is pathognomonic for chromoblastomycosis. Sclerotic bodies are thick-walled, brownish, spherical cells that divide by septation rather than budding, and they are characteristic of this chronic subcutaneous infection.

Why not the others?

• b) Aspergillosis: Caused by hyaline (non-pigmented) molds like Aspergillus; sclerotic bodies are not seen.

• c) Sporotrichosis: Caused by Sporothrix schenckii, which is not dematiaceous; it appears as yeast-like cells or cigar-shaped bodies in tissue, not sclerotic bodies.

• d) Candidiasis: Caused by Candida species, which are yeasts or hyaline molds; they do not produce sclerotic bodies or dematiaceous pigments.

The echinocandins (e.g., caspofungin, micafungin, anidulafungin) are a class of antifungal agents that specifically inhibit the synthesis of β-(1,3)-D-glucan, a critical component of the fungal cell wall. This disruption weakens the cell wall, leading to osmotic instability and cell lysis. Echinocandins are particularly effective against Candida and Aspergillus species.

Why not the others?

• a) Polyenes (e.g., amphotericin B): Bind to ergosterol in the fungal cell membrane, forming pores that cause leakage of cellular contents.

• b) Azoles (e.g., fluconazole, voriconazole): Inhibit ergosterol synthesis by targeting lanosterol 14-α-demethylase.

• d) Pyrimidine analogs (e.g., flucytosine): Interfere with fungal RNA and DNA synthesis after conversion to 5-fluorouracil.

The hair perforation test is specifically used to differentiate between certain dermatophytes, particularly:

• Trichophyton mentagrophytes: Positive for hair perforation. It produces enzyme-like structures that create wedge-shaped erosions in hair shafts when tested in vitro.

• Trichophyton rubrum: Negative for hair perforation and does not cause these erosions.

While the test is primarily used to distinguish between T. mentagrophytes and T. rubrum, it can also help differentiate other dermatophytes (e.g., Microsporum species generally do not perforate hair in the same way). However, its main application is within the Trichophyton genus.

Why not the others?

• b) Candida species: Identified by germ tube tests, carbohydrate assimilation, or chromogenic agar—not hair perforation.

• c) Aspergillus species: Distinguished by colony morphology, conidial head structure, and temperature tolerance—hair perforation is irrelevant.

• d) Cryptococcus species: Identified by capsule production (India ink), urease test, and phenol oxidase test—hair perforation is not used.

The fungus formerly known as Penicillium marneffei has been reclassified based on molecular and genetic studies. Its teleomorph (sexual form) is now correctly classified as Talaromyces marneffei. This dimorphic fungus is a significant pathogen in Southeast Asia, causing systemic infections in immunocompromised patients (e.g., those with AIDS). The reclassification reflects its phylogenetic placement within the genus Talaromyces rather than Penicillium.

Why not the others?

• b) Aspergillus nidulans: Has a teleomorph known as Emericella nidulans, not related to Talaromyces.

• c) Pseudallescheria boydii: Now reclassified as Scedosporium apiospermum in its asexual form, with Pseudallescheria as its teleomorph (but unrelated to Talaromyces).

• d) Geotrichum candidum: A yeast-like fungus with no teleomorph named Talaromyces.

• Reverse CAMP test is used to identify Clostridium perfringens, particularly the type B beta-toxin activity.

• Principle:

  • Clostridium perfringens produces a beta-toxin that inhibits hemolysis caused by Staphylococcus aureus on blood agar.

  • This produces an “inverse” or arrowhead pattern compared to the standard CAMP test used for Streptococcus agalactiae.

Other options:

• Streptococcus agalactiae → identified using standard CAMP test, not reverse

• Candida albicans → identified by germ tube test

• Cryptococcus neoformans → identified by India ink, urease, or cryptococcal antigen

The India ink preparation is a classic rapid test used to detect the capsule of Cryptococcus neoformans in cerebrospinal fluid (CSF) or other clinical specimens. The capsule appears as a clear halo around the yeast cell against the dark background of the ink, due to the capsule’s ability to exclude the ink particles. This is a key diagnostic feature for cryptococcal meningitis, especially in immunocompromised patients (e.g., those with AIDS).

Why not the others?

• a) Candida albicans: Does not produce a true capsule (though it may have a slime layer) and is not detected by India ink.

• c) Histoplasma capsulatum: Is a small intracellular yeast that lacks a capsule; its name refers to its appearance within macrophages, not a capsule.

• d) Coccidioides immitis: In tissue, it forms spherules (not yeast with capsules) and is not visible with India ink.

The primary and most serious fungal infection caused by Cryptococcus neoformans (and C. gattii) is cryptococcal meningitis. This occurs when the yeast disseminates from the lungs to the central nervous system (CNS), leading to inflammation of the meninges. It is a life-threatening condition, especially in immunocompromised individuals (e.g., those with HIV/AIDS, organ transplant recipients).

Why not the others?

• a) Skin tinea: Cryptococcus does not cause typical dermatophytosis (ringworm); skin lesions may occur in disseminated disease but are not primary.

• c) Nail infection: Onychomycosis is primarily caused by dermatophytes or Candida, not Cryptococcus.

• d) Pulmonary infection only: While inhalation of cryptococcal spores can cause pulmonary infection (e.g., cryptococcal pneumonia), the fungus has a strong tropism for the CNS, and disseminated meningitis is the hallmark of severe disease.

The KOH (potassium hydroxide) preparation is a simple, rapid test used to detect fungal elements (e.g., hyphae, spores, yeast cells) in skin, hair, or nail scrapings. KOH dissolves keratin and non-fungal cellular material in the specimen, clearing the background and making fungal structures more visible under a microscope. It is the primary method for diagnosing superficial fungal infections like dermatophytosis (ringworm), tinea versicolor, and candidiasis.

Why not the others?

• a) Gram stain: Used primarily for bacteria; fungi may stain but are not clearly distinguished in keratin-rich samples.

• b) India ink: Used to detect the capsule of Cryptococcus neoformans in cerebrospinal fluid, not skin scrapings.

• d) Acid-fast stain: Used for Mycobacterium species (e.g., tuberculosis), not fungi.

The clinical presentation of rhinocerebral infection with black, necrotic turbinates in a diabetic patient (often with ketoacidosis) is classic for mucormycosis. The direct KOH preparation showing broad, nonseptate (or sparsely septate) hyphae with right-angle branching is pathognomonic for fungi in the Zygomycetes class (e.g., Rhizopus, Mucor, Lichtheimia). These angioinvasive fungi cause rapid tissue necrosis and are associated with high mortality.

Why not the others?

• a) Aspergillus fumigatus: Causes invasive sinusitis but exhibits septate hyphae with acute-angle branching (not broad/nonseptate). It is less associated with black eschars and diabetic ketoacidosis.

• b) Candida albicans: A yeast that forms pseudohyphae or true hyphae (which are septate) but does not cause rhinocerebral mucormycosis-like presentations.

• d) Pseudallescheria boydii: Causes fungal sinusitis but has septate hyphae resembling Aspergillus; it is not typically linked to diabetic ketoacidosis or broad nonseptate hyphae.

The reverse CAMP test is a microbiological assay used to identify Clostridium perfringens (and occasionally other clostridia). Here’s how it works:

• A streak of Streptococcus agalactiae (Group B Strep) is made on a blood agar plate.

• Clostridium perfringens is streaked perpendicular to it but not touching.

• After anaerobic incubation, C. perfringens produces a rectangle-shaped zone of enhanced hemolysis where the two streaks align, due to the interaction between the alpha-toxin of C. perfringens and the CAMP factor of S. agalactiae.

This is distinct from the standard CAMP test (used to identify S. agalactiae), where a Staphylococcus aureus streak is used.

The other options are incorrect:

• b) Streptococcus agalactiae: Identified by the standard CAMP test.

• c) Candida albicans: Identified by germ tube tests, chromogenic agar, or biochemical assays.

• d) Cryptococcus neoformans: Identified by urease test, capsule production, and growth on selective media.

Sporotrichosis, caused by the dimorphic fungus Sporothrix schenckii, is classically known as “rose gardener’s disease”. It typically presents with subcutaneous nodules that form along lymphatic channels (lymphocutaneous sporotrichosis) following traumatic inoculation (e.g., from thorns, splinters, or soil). The nodules may ulcerate and drain, creating a linear pattern from the initial site of infection toward regional lymph nodes.

Why not the others?

• b) Coccidioidomycosis: Acquired via inhalation of spores, causing respiratory illness (e.g., Valley fever), not subcutaneous lymphatic spread.

• c) Histoplasmosis: Also inhaled, causing pulmonary or disseminated disease, without subcutaneous lymphatic involvement.

• d) Blastomycosis: Primarily a respiratory infection that may disseminate to skin (but not typically along lymphatics).

Coccidioides immitis (and Coccidioides posadasii) is a dimorphic fungus that produces arthroconidia (also called arthrospores) in its environmental mold phase. These are barrel-shaped, asexual spores formed by the fragmentation of hyphae. When these lightweight, airborne arthroconidia are inhaled, they can cause coccidioidomycosis (Valley Fever). This method of sporulation is a key characteristic of Coccidioides species.

Why not the others?

• b) Blastomyces dermatitidis: Produces conidia (not arthroconidia) from conidiophores in its mold form.

• c) Candida albicans: Reproduces by budding (blastoconidia) and may form pseudohyphae or true hyphae, but not arthroconidia.

• d) Aspergillus fumigatus: Produces conidia (asexual spores) from conidiophores, not arthroconidia.

“Black dot” tinea capitis is a form of scalp ringworm where hair shafts break off at the scalp surface, leaving behind dark, stubbly remnants that resemble black dots. This is primarily caused by Trichophyton tonsurans, which is an endothrix fungus. This means the spores of the fungus are found inside the hair shaft, leading to fragility and breakage.

Why not the others?

• b) Microsporum canis: Causes tinea capitis but typically results in hair breaking off above the scalp (ectothrix infection) and may fluoresce green under Wood’s lamp. It does not produce the “black dot” appearance.

• c) Epidermophyton floccosum: Infects skin and nails but does not cause hair infections (tinea capitis).

• d) Candida albicans: Causes oral thrush or vaginitis but is not a common cause of tinea capitis or “black dot” patterns.

Epidermophyton floccosum is a dermatophyte that primarily produces smooth-walled, club-shaped (clavate) macroconidia in clusters. These macroconidia are typically multicellular (with 2–4 cells) and are borne directly from the hyphae. Notably, E. floccosum does not produce microconidia, which helps distinguish it from other dermatophytes like Trichophyton or Microsporum.

Why not the others?

• b) Arthroconidia: Produced by fungi like Coccidioides (environmental form) or some dermatophytes in old cultures, but not typical for E. floccosum.

• c) Blastoconidia: Budding yeast cells seen in Candida species.

• d) Sporangiospores: Asexual spores produced by zygomycetes (e.g., Mucor, Rhizopus) within sporangia.

Fusarium species are opportunistic molds that are particularly dangerous for severely neutropenic patients (e.g., those undergoing chemotherapy or with hematologic malignancies). They can cause:

• Disseminated infections: With bloodborne spread (fungemia), skin lesions, and multiorgan involvement.

• High mortality: Due to the aggressiveness of the fungus and the patient’s compromised immune state.

While Fusarium can cause localized infections like keratitis (eye infections) or onychomycosis (nail infections) in immunocompetent individuals, its most significant impact is in immunocompromised hosts, where it leads to severe systemic disease.

Why not the others?

• a) Skin only: Fusarium can cause skin infections but is not limited to skin; it is notorious for dissemination.

• b) Allergies only: Not a primary concern; Fusarium is more associated with invasive disease than allergies.

• d) Nail infections only: While it can cause onychomycosis, this is less common than dermatophytes and not the major clinical concern.

Tinea capitis is a fungal infection specifically involving the scalp and hair shafts. It is primarily caused by dermatophytes from the genera Trichophyton and Microsporum. Common symptoms include scaling, hair loss, and sometimes inflammation (e.g., kerion formation). The term “tinea” denotes a fungal infection, and “capitis” refers to the head.

Why not the others?

• a) Nails: Fungal nail infections are called tinea unguium (or onychomycosis).

• b) Skin of abdomen: Ringworm on the body is termed tinea corporis.

• d) Feet: Athlete’s foot is known as tinea pedis.

Tinea nigra is a superficial, non-inflammatory fungal infection that presents as brown to black macules on the palms or soles. It is caused by Hortaea werneckii (formerly classified as Exophiala werneckii or Cladosporium werneckii), a dematiaceous (dark-pigmented) fungus. The infection is typically asymptomatic and is acquired from contact with soil, wood, or decaying vegetation in tropical and subtropical regions.

Why not the others?

• b) Trichophyton rubrum: Causes dermatophytoses (e.g., athlete’s foot, ringworm) but not tinea nigra.

• c) Malassezia furfur: Causes tinea versicolor (pityriasis versicolor), presenting as hypopigmented or hyperpigmented patches on the trunk, not palmar black macules.

• d) Piedraia hortae: Causes black piedra, a hair infection characterized by hard, dark nodules on the hair shaft, not skin macules.

• Cornmeal agar with Tween 80 is commonly used to study the morphology of Candida species and to enhance the formation of chlamydospores, blastoconidia, pseudohyphae, and true hyphae.

• Sabouraud dextrose agar (SDA): General-purpose medium for isolating fungi but not specific for inducing special structures.

• Blood agar / Chocolate agar: Primarily used for bacteria; not suitable for enhancing fungal reproductive structures

Alternaria species are dematiaceous (dark-pigmented) molds commonly found in the environment (e.g., soil, plants, air) and are frequent laboratory contaminants. However, they can act as opportunistic pathogens, causing:

• Onychomycosis (fungal nail infections)

• Keratitis (fungal eye infections, particularly in contact lens wearers or after trauma)

• Cutaneous and subcutaneous infections in immunocompromised hosts.

Why not the others?

• b) Cryptococcus neoformans: Primarily a pathogen (causing cryptococcosis) and not typically considered a common lab contaminant; it is associated with bird droppings.

• c) Sporothrix schenckii: A dimorphic fungus that causes sporotrichosis (e.g., rose gardener’s disease) and is not a common contaminant.

• d) Histoplasma capsulatum: A dimorphic fungus that causes histoplasmosis and is acquired from bat/bird droppings; it is not a lab contaminant.

• The string test (also known as the “germ tube” test in a different context, but here it refers to a physical test) is specifically used for the presumptive identification of Candida albicans.

• Procedure: A small sample of a yeast colony is mixed with a drop of serum on a microscope slide and incubated at 37°C for 2-3 hours. If germ tubes (short, filamentous outgrowths without constriction at the base) are produced, it is a positive test. Candida albicans (and the rare Candida dubliniensis) are the common yeasts that produce true germ tubes.

• This test is rapid and helps differentiate C. albicans from other Candida species.

The other options are incorrect:

• b) Cryptococcus species: Identified by urease test, capsule production (India ink), and growth on birdseed agar.

• c) Trichophyton species: Identified by microscopic morphology of macroconidia and microconidia on fungal cultures.

• d) Aspergillus species: Identified by colony morphology and microscopic structures (conidiophores, vesicles, conidia).

The antigen detection test (e.g., using enzyme immunoassay) in urine and serum is highly sensitive and specific for diagnosing histoplasmosis, especially in disseminated or acute pulmonary cases. Histoplasma capsulatum releases polysaccharide antigens that can be detected in body fluids, making this test a cornerstone for rapid diagnosis, particularly in immunocompromised patients (e.g., those with AIDS).

Why not the others?

• a) Candidiasis: Diagnosis relies on culture, histopathology, or β-D-glucan testing; antigen detection is not routinely used.

• b) Aspergillosis: Diagnosed via galactomannan antigen testing in serum or bronchoalveolar lavage (BAL), but it is less sensitive/specific than histoplasma antigen testing and is not typically detected in urine.

• d) Dermatophytosis: Diagnosed by KOH microscopy and culture; no antigen tests are available.

Calcofluor white is a fluorescent dye that binds to chitin and cellulose in fungal cell walls. When added to a KOH preparation, it causes fungal elements (hyphae, spores, yeast cells) to fluoresce a bright apple-green or blue-white color under an ultraviolet (UV) or fluorescent microscope. This greatly enhances the sensitivity and visibility of fungi, especially in paucity or against a complex background.

Why not the others?

• a) Enhances color contrast: While it improves visibility, the key mechanism is fluorescence, not simple color contrast.

• c) Kills bacteria: Calcofluor white is not antimicrobial; it is a stain specific to fungal structures.

• d) Preserves specimen: It does not preserve the specimen; KOH clears debris, and calcofluor white adds fluorescence for detection.

Chromoblastomycosis is a chronic subcutaneous infection caused by dematiaceous molds (dark-pigmented fungi due to melanin in their cell walls). Common causative agents include:

• Fonsecaea pedrosoi

• Cladophialophora carrionii

• Phialophora verrucosa

These fungi produce characteristic sclerotic bodies (also called muriform bodies or copper-penny bodies) in tissue—thick-walled, brownish, spherical cells that divide by septation. The infection often results from traumatic inoculation (e.g., by thorns or splinters) and presents as warty, scaly lesions.

Why not the others?

• a) Hyaline molds (e.g., Aspergillus, Fusarium): Lack melanin and cause infections like aspergillosis or hyalohyphomycosis, not chromoblastomycosis.

• c) Yeasts (e.g., Candida, Cryptococcus): Do not produce sclerotic bodies or cause this type of infection.

• d) Zygomycetes (e.g., Mucor, Rhizopus): Are non-pigmented and cause mucormycosis, characterized by broad, nonseptate hyphae.

The “molar tooth” appearance refers to the rough, irregular, and cratered colonies of Actinomyces israelii when grown anaerobically on agar plates. This Gram-positive, anaerobic bacterium is often confused with fungi due to its filamentous, branching morphology (resembling fungal hyphae) and its ability to cause chronic granulomatous infections. However, it is a bacterium, not a fungus, and is part of the normal flora of the mouth and gastrointestinal tract.

Why not the others?

• b) Nocardia asteroides: Forms chalky, wrinkled colonies but not typically “molar tooth”; it is aerobic and partially acid-fast.

• c) Clostridium perfringens: Forms smooth, double-zone hemolytic colonies on blood agar, not molar tooth.

• d) Lactobacillus species: Forms small, smooth colonies and is not filamentous or confused with fungi.

Dermatophytes are fungi that infect keratinized tissues (skin, hair, nails), and only some species fluoresce under a Wood’s lamp (ultraviolet light at 365 nm). The fluorescence is primarily associated with:

• Microsporum species (e.g., Microsporum canis, Microsporum audouinii): Infect hair and produce an apple-green fluorescence due to metabolites and fungal elements on the hair shaft.

• Trichophyton species (e.g., Trichophyton schoenleinii, a rare cause of favus): May produce a pale green or blue-white fluorescence.

• Epidermophyton floccosum: Does not typically fluoresce, but it is grouped with dermatophytes that are evaluated under Wood’s lamp.

However, note that not all dermatophytes fluoresce. For example:

• Trichophyton rubrum (the most common dermatophyte) does not fluoresce.

• Epidermophyton floccosum rarely fluoresces.

Why not the others?

• a) Candida species: Do not fluoresce under Wood’s lamp.

• b) Trichophyton only: Incorrect, as only specific Trichophyton species fluoresce, and Microsporum is a major contributor.

• d) Aspergillus species: Environmental molds that do not fluoresce in a diagnostic context for skin/hair infections.

Macroconidia with echinulations (rough, spiny walls) that are described as “sleeve” macroconidia due to their elongated, spindle-shaped appearance with tapered ends are characteristic of Microsporum canis. These structures are multicellular (typically with 5–15 cells) and are a key microscopic feature for identifying this dermatophyte, which causes infections like tinea capitis and tinea corporis.

Why not the others?

• b) Trichophyton rubrum: Produces smooth-walled, pencil-shaped macroconidia (if present at all; microconidia are more common).

• c) Epidermophyton floccosum: Forms smooth-walled, club-shaped (clavate) macroconidia in clusters, without echinulations.

• d) Candida albicans: A yeast that produces budding cells and pseudohyphae, not macroconidia.

Dermatophytosis (e.g., ringworm, athlete’s foot, jock itch) is caused by fungi that infect keratinized tissues (skin, hair, nails). The most common causative agents belong to three genera:

1. Trichophyton (e.g., T. rubrum, T. mentagrophytes)

2. Microsporum (e.g., M. canis, M. audouinii)

3. Epidermophyton (e.g., E. floccosum)

These dermatophytes are specialized pathogens that produce keratinases, allowing them to invade and thrive in keratin-rich environments.

Why not the others?

• a) Histoplasma and Blastomyces: Dimorphic fungi that cause systemic infections (e.g., histoplasmosis, blastomycosis), not superficial dermatophytosis.

• b) Penicillium and Aspergillus: Environmental molds that may cause opportunistic infections (e.g., aspergillosis) but are not dermatophytes.

• d) Candida and Cryptococcus: Yeasts that cause candidiasis (e.g., thrush) or cryptococcosis (meningitis), but they are not primary causes of classic dermatophytosis.

Aspergillus fumigatus is a ubiquitous mold commonly found in the environment (e.g., soil, decaying organic matter, air). It is a frequent laboratory contaminant and a major allergen (e.g., causing allergic bronchopulmonary aspergillosis, asthma). However, in immunocompromised hosts (e.g., neutropenic patients, those on immunosuppressive therapy), it can cause severe invasive aspergillosis, often affecting the lungs, sinuses, or other organs.

Why not the others?

• b) Cryptococcus neoformans: Primarily a pathogen (associated with pigeon droppings) and not typically considered a common contaminant or allergen.

• c) Candida albicans: Part of the normal human microbiota and an opportunistic pathogen, but not a common environmental contaminant or allergen.

• d) Blastomyces dermatitidis: A dimorphic fungus that causes blastomycosis (acquired from environmental sources like soil) but is not a common contaminant or allergen.

The satellite phenomenon refers to the observation where a fastidious organism (like Haemophilus influenzae) grows only in the vicinity of another organism (like Staphylococcus aureus) that produces a necessary growth factor.

• Haemophilus influenzae requires both V factor (NAD) and X factor (hemin) for growth.

• Staphylococcus aureus produces V factor (NAD) as a metabolic byproduct and releases it into the surrounding medium as it grows.

• On a blood agar plate, Haemophilus influenzae will form small colonies that “satellite” around the colonies of S. aureus. It will not grow in areas of the plate where the factor is not available.

The other options are incorrect:

• b) Staphylococcus aureus: This is the organism that provides the factor for the satellite phenomenon; it is not the one that exhibits it.

• c) Cryptococcus neoformans: This is a yeast and its growth is not characterized by the satellite phenomenon.

• d) Candida albicans: This is also a yeast and does not exhibit satelliting.

The description of nonseptate (aseptate), ribbon-like hyphae with right-angle branching is classic for fungi in the Zygomycetes class (e.g., Mucor, Rhizopus, Lichtheimia). These molds cause mucormycosis and exhibit:

• Broad, nonseptate (or sparsely septate) hyphae that appear ribbon-like and irregular in width.

• Right-angle (90°) branching in tissue specimens, which helps distinguish them from other molds like Aspergillus.

Why not the others?

• a) Aspergillus species: Have septate hyphae with acute-angle (45°) branching.

• c) Dermatophytes: Have septate hyphae but do not typically show right-angle branching in tissue.

• d) Fusarium species: Have septate hyphae and may branch at acute angles, similar to Aspergillus.

While all fungal infections can be more severe in immunocompromised patients, candidiasis (particularly invasive candidiasis, such as candidemia or disseminated disease) is most strongly associated with neutropenia and immunocompromised states (e.g., chemotherapy, organ transplantation, HIV/AIDS). Neutropenia impairs the body’s ability to clear fungal pathogens, and Candida species (especially C. albicans and non-albicans species) are common opportunistic pathogens that thrive in this setting.

The other options are endemic fungal infections that can worsen with immunosuppression but are not as directly linked to neutropenia:

• b) Histoplasmosis: Associated with cell-mediated immune defects (e.g., AIDS).

• c) Blastomycosis: Less tied to neutropenia; often affects immunocompetent hosts.

• d) Coccidioidomycosis: Risk increases with immunosuppression but is not specifically neutropenia-driven.

Tinea versicolor (also known as pityriasis versicolor) is a common superficial fungal infection characterized by hypopigmented or hyperpigmented patches on the skin, often on the trunk, neck, and arms. It is caused by Malassezia furfur (and other Malassezia species), a lipophilic yeast that is part of the normal skin flora but can overgrow in warm, humid conditions or due to host factors (e.g., oily skin, immunosuppression).

Why not the others?

• a) Epidermophyton floccosum: A dermatophyte that causes infections like tinea cruris (jock itch) or tinea pedis (athlete’s foot), not tinea versicolor.

• c) Candida albicans: Causes candidiasis (e.g., thrush, vaginitis) and intertrigo, but not the characteristic patches of tinea versicolor.

• d) Aspergillus niger: An environmental mold that may cause opportunistic infections (e.g., otomycosis) but is not associated with tinea versicolor.

• Cryptococcus neoformans is a urease-positive yeast. This property, along with its polysaccharide capsule, helps distinguish it from other yeasts like Candida.

• It’s an important diagnostic feature, especially when differentiating it in CSF cultures.

• Other options:

  • Candida glabrata → Urease negative.

  • Aspergillus fumigatus → Mold, urease not used for its identification.

  • Mucor species → Zygomycetes, not identified by urease test.

Onychomycosis is confirmed by:
a) Blood culture
b) Antibody test
c) Clinical appearance and lab testing
d) PCR only

Histoplasmosis, caused by the dimorphic fungus Histoplasma capsulatum, is characterized by:

• Granulomatous inflammation: The infection often triggers a robust cell-mediated immune response, leading to the formation of granulomas (organized collections of macrophages and other immune cells) in the lungs and other organs.

• Pulmonary disease resembling tuberculosis: Primary pulmonary histoplasmosis can present with symptoms and radiographic findings (e.g., hilar lymphadenopathy, lung infiltrates, calcifications) that mimic tuberculosis. Chronic or disseminated forms may also resemble TB clinically.

Why not the others?

• a) Candidiasis: Typically causes mucosal or bloodstream infections without granuloma formation; it does not resemble TB.

• b) Cryptococcosis: Often causes meningitis or pulmonary lesions but is associated with gelatinous capsules and lack of granulomas in immunocompromised hosts (though granulomas can form in immunocompetent individuals).

• d) Tinea versicolor: A superficial skin infection caused by Malassezia species, with no pulmonary involvement or granulomatous reaction.

The description is classic for a dimorphic fungus:

• At 25°C (room temperature), it grows as a mold (white, cottony appearance).

• At 37°C (body temperature), it converts to a yeast form (smooth, cream-colored colonies).

This temperature-dependent transition is a defining characteristic of dimorphic fungi, such as Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Sporothrix schenckii, and Talaromyces marneffei.

Why not the others?

• a) A dermatophyte: Grows only as a mold at both temperatures (no yeast phase).

• c) A zygomycete: Grows as a mold with nonseptate hyphae at both temperatures (e.g., Mucor, Rhizopus).

• d) A dematiaceous fungus: Pigmented molds that remain molds at both temperatures (e.g., Exophiala, Cladophialophora).

Cryptococcus neoformans is a yeast that has two key identifying features:

1. Urease-positive: It produces the enzyme urease, which hydrolyzes urea to ammonia and carbon dioxide, leading to a color change in urea agar.

2. Capsule production: It is surrounded by a wide, polysaccharide capsule, which is a major virulence factor. The capsule can be visualized using India ink or mucicarmine staining.

These characteristics help differentiate it from other fungi:

• b) Candida albicans: Is urease-negative and does not produce a true capsule (though it may have a slime layer).

• c) Histoplasma capsulatum: Is urease-negative and does not have a capsule (its name refers to its appearance within macrophages, not an actual capsule).

• d) Aspergillus fumigatus: Is a mold that is urease-negative and does not produce a capsule.

Coccidioides immitis (and Coccidioides posadasii) is a dimorphic fungus that forms spherules in the lungs and other tissues during infection. In its parasitic phase (within the human host at 37°C), it develops large, thick-walled spherules that contain numerous endospores. When the spherules rupture, they release these endospores, which can disseminate and form new spherules. This unique structure is pathognomonic for coccidioidomycosis (Valley Fever).

Why not the others?

• a) Histoplasma capsulatum: Forms small, oval yeast cells (2–4 μm) within macrophages in tissues, not spherules.

• c) Blastomyces dermatitidis: Appears as large, broad-based budding yeast cells (8–15 μm) in tissues.

• d) Sporothrix schenckii: Typically forms cigar-shaped yeast cells in tissues after traumatic inoculation (e.g., from rose thorns).

The “spaghetti and meatball” appearance is a classic description of the microscopic view of Malassezia furfur (now also classified as Malassezia globosa and others) in a potassium hydroxide (KOH) preparation. This refers to the short, curved hyphal forms (“spaghetti”) and clusters of round, thick-walled yeast cells (“meatballs”) typically seen in skin scrapings from patients with tinea versicolor (pityriasis versicolor).

The other options are incorrect:

• b) Candida albicans: Appears as budding yeasts with or without pseudohyphae (germ tubes if positive), not the “spaghetti and meatball” pattern.

• c) Trichophyton rubrum: Shows septate hyphae and/or arthroconidia in KOH mounts from skin or nail samples.

• d) Microsporum canis: Reveals hyphae and arthroconidia in hair samples, sometimes with ectothrix spores, but not the distinctive “spaghetti and meatball” morphology.

The term “spaghetti and meatball” is a classic microscopic description for Malassezia furfur (and other Malassezia species) in a potassium hydroxide (KOH) preparation of skin scrapings. It refers to:

• Short, curved hyphal forms (“spaghetti”)

• Clusters of round, thick-walled yeast cells (“meatballs”)

This pattern is pathognomonic for tinea versicolor (pityriasis versicolor), a superficial skin infection.

Why not the others?

• a) Dermatophytes: Appear as long, branching, septate hyphae and/or arthroconidia (e.g., in ringworm), not the “spaghetti and meatball” pattern.

• b) Candida forms: Show budding yeast cells, pseudohyphae, or true hyphae (e.g., in thrush), but lack the distinct clustered yeast cells.

• d) Aspergillus conidiophores: Display septate hyphae with specialized conidiophores and vesicles bearing conidia, not the Malassezia-specific morphology.

Malassezia furfur is a lipophilic yeast that requires long-chain fatty acids for growth. Standard fungal media like Sabouraud dextrose agar (SDA) do not support its growth unless supplemented with lipids. Therefore, olive oil–enriched medium (e.g., SDA with olive oil or specialized media like Dixon’s agar) is used to culture M. furfur. The oil provides the necessary lipids for proliferation.

Why not the others?

• a) Sabouraud dextrose agar (SDA): Without lipid supplementation, M. furfur typically will not grow on plain SDA.

• c) Blood agar and d) Chocolate agar: These are nutrient-rich media for bacteria or fastidious organisms but lack the specific lipid requirements for M. furfur.

The exoantigen immunodiffusion test is a laboratory method used to confirm the identity of dimorphic fungi such as:

• Histoplasma capsulatum (detects H and M antigens)

• Blastomyces dermatitidis (detects A antigen)

• Coccidioides immitis (detects specific antigens)

This test involves extracting antigens from the fungal culture (mold form) and allowing them to diffuse against known antisera in an agar gel. The formation of precipitin lines confirms the species. It is highly specific and avoids the need for converting the mold to the yeast phase (which can be slow or hazardous).

Why not the others?

• a) Dermatophytes: Identified by microscopic morphology (e.g., macroconidia) and culture characteristics, not exoantigen tests.

• c) Candida species: Identified through biochemical tests (e.g., germ tube, carbohydrate assimilation) or chromogenic agar.

• d) Cryptococcus species: Identified by capsule production (India ink), urease test, and phenol oxidase test (birdseed agar).

Chromoblastomycosis is a chronic subcutaneous infection characterized by scaly, wart-like lesions. It is caused by dematiaceous fungi (dark-walled molds), which contain melanin in their cell walls. Common causative agents include Fonsecaea, Cladophialophora, and Phialophora species. These fungi are identified by their dark pigmentation and characteristic sclerotic bodies (copper-colored, thick-walled cells) in tissue.

The other options are incorrect:

• a) Hyaline molds (e.g., Aspergillus, Fusarium) lack melanin and are not associated with chromoblastomycosis.

• c) Yeasts (e.g., Candida, Cryptococcus) do not cause this condition.

• d) Zygomycetes (e.g., Mucor, Rhizopus) are non-pigmented and cause mucormycosis, not chromoblastomycosis.

Sabouraud dextrose agar (SDA) is a common fungal culture medium. When supplemented with antibiotics such as:

• Chloramphenicol: Inhibits bacterial growth.

• Cycloheximide: Suppresses saprophytic (environmental) molds and non-pathogenic fungi that may overgrow slower-growing pathogenic fungi.

This selective formulation (e.g., SDA with chloramphenicol and cycloheximide) is specifically designed to isolate pathogenic fungi like dermatophytes, Candida, and dimorphic fungi (Histoplasma, Blastomyces, etc.) from contaminated specimens.

Why not the others?

• a) Blood agar: Supports growth of both bacteria and fungi but lacks selective inhibitors for saprophytic molds.

• b) MacConkey agar: Selective for Gram-negative bacteria and inhibits Gram-positive bacteria and fungi.

• d) Thayer-Martin agar: Used for isolating Neisseria gonorrhoeae and N. meningitidis by suppressing bacteria and fungi but not saprophytic molds.

• Cryptococcosis is caused by the yeast Cryptococcus neoformans and Cryptococcus gattii.

• The primary ecological niche for C. neoformans is soil contaminated with bird droppings, particularly pigeon excreta. The fungus thrives in the nitrogen-rich environment provided by the guano. Humans typically acquire the infection by inhaling aerosolized spores from contaminated soil.

The other options are incorrect:

• a) Coccidioidomycosis: Associated with desert soils in the southwestern United States and parts of Central and South America.

• c) Aspergillosis: Caused by Aspergillus species, which are ubiquitous molds found in soil, decaying vegetation, and hay.

• d) Blastomycosis: Associated with moist soil and decomposing organic matter like wood and leaves, often in areas around the Ohio and Mississippi River valleys and the Great Lakes.

• Tuberculate macroconidia refer to large spores with a thick, rough, or spiny wall that projects in a finger-like manner. This is a classic and defining microscopic characteristic of Histoplasma capsulatum when it is in its mold (mycelial) form.

• The other options are incorrect:

  • b) Aspergillus niger produces characteristic radiating chains of conidia from a vesicle, not tuberculate macroconidia.

  • c) Microsporum canis produces spindle-shaped, thick-walled macroconidia with a rough or spiny surface, but these are not typically described as “tuberculate” in the same distinctive manner as H. capsulatum.

  • d) Rhizopus species are zygomycetes that produce sporangiospores within sac-like sporangia and do not produce macroconidia.

The production of a brown to black pigment on birdseed agar (or caffeic acid agar) is a key diagnostic feature of Cryptococcus neoformans. This pigment results from the oxidation of phenolic compounds (e.g., caffeic acid) in the agar by the enzyme phenol oxidase (laccase) produced by the yeast, leading to melanin formation. This test is highly specific for differentiating C. neoformans from other yeasts.

Why not the others?

• a) Candida albicans: Does not produce melanin; it appears as creamy white colonies and is identified by germ tube tests or chromogenic agar.

• c) Histoplasma capsulatum: A dimorphic fungus that does not grow typically on birdseed agar; it requires specialized media and converts to yeast form at 37°C.

• d) Blastomyces dermatitidis: Also dimorphic and does not produce pigment on birdseed agar; it is identified by its broad-based budding yeast morphology in tissues.

Onychomycosis (fungal nail infection) is most commonly caused by dermatophytes, and Epidermophyton floccosum is one of the primary dermatophyte genera (alongside Trichophyton and Microsporum) associated with this condition. Specifically:

• Trichophyton rubrum is the most frequent cause of onychomycosis worldwide.

• Epidermophyton floccosum is also a common culprit, particularly for nail and skin infections (e.g., tinea pedis, tinea cruris).

Why not the others?

• a) Histoplasma capsulatum: A dimorphic fungus that causes systemic histoplasmosis, not nail infections.

• c) Cryptococcus neoformans: An encapsulated yeast that causes cryptococcosis (e.g., meningitis) and is rarely associated with onychomycosis.

• d) Rhizopus species: A zygomycete (mucormycete) that causes mucormycosis, typically in immunocompromised hosts, and does not infect nails.

The exoantigen test is an immunodiffusion technique used for the definitive identification of dimorphic fungi such as Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides immitis. This test detects specific fungal antigens (e.g., H and M antigens for Histoplasma, A antigen for Blastomyces) released into the culture supernatant. When these antigens react with known antibodies in immunodiffusion plates, precipitin lines form, confirming the identity of the fungus. It is particularly useful for confirming the identity of these pathogens without requiring conversion to the yeast phase, which can be time-consuming or hazardous.

Why not the others?

• a) Dermatophytes: Identified primarily by microscopic morphology (macroconidia/microconidia) and culture characteristics, not exoantigen tests.

• c) Candida species: Identified through biochemical tests (e.g., germ tube, carbohydrate assimilation) or chromogenic agar.

• d) Cryptococcus species: Identified by capsule production (India ink), urease test, and phenol oxidase test (birdseed agar).

Actinomyces israelii is a bacterium (specifically, a filamentous, Gram-positive anaerobic bacterium) and is a common cause of actinomycotic mycetoma (bacterial mycetoma), not eumycotic mycetoma (fungal mycetoma). Mycetoma can be classified based on the causative agent:

• Eumycotic mycetoma: Caused by true fungi (eumycetes), such as Madurella mycetomatis, Pseudallescheria boydii, and Exophiala jeanselmei.

• Actinomycotic mycetoma: Caused by filamentous bacteria, such as Actinomyces israelii, Nocardia species, and Streptomyces somaliensis.

Why the others are incorrect:

• a) Madurella mycetomatis: A common fungal cause of eumycotic mycetoma, often producing black grains.

• b) Pseudallescheria boydii: A frequent fungal agent of eumycotic mycetoma, especially in temperate regions.

• d) Exophiala jeanselmei: A dematiaceous fungus that can cause eumycotic mycetoma with black grains.

Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum. The primary mode of transmission is the inhalation of spores (conidia) from disturbed soil or environments contaminated with bat or bird droppings (e.g., caves, chicken coops, old buildings). The fungus thrives in nitrogen-rich guano, and spores become airborne during activities like cleaning or excavation.

Why not the others?

• b) Candidiasis: Caused by Candida species (e.g., C. albicans), which are part of the normal human microbiota and typically cause opportunistic infections (e.g., thrush, vaginitis), not acquired from soil or droppings.

• c) Sporotrichosis: Caused by Sporothrix schenckii, often acquired through traumatic inoculation into the skin (e.g., from rose thorns or sphagnum moss), not inhalation of spores from droppings.

• d) Dermatophytosis: Caused by dermatophytes (e.g., Trichophyton, Microsporum), which are transmitted via direct contact with infected humans, animals, or fomites, not from bird/bat droppings.

Cryptococcal meningitis, caused by Cryptococcus neoformans or Cryptococcus gattii, is one of the most common life-threatening fungal infections in AIDS patients (particularly those with CD4 counts <100 cells/μL). The fungus enters the body via inhalation and can disseminate to the central nervous system, leading to meningitis. Its high prevalence in immunocompromised individuals is due to impaired cell-mediated immunity, which is critical for controlling cryptococcal infection.

Why not the others?

• b) Blastomycosis: More common in immunocompetent hosts and not specifically linked to AIDS.

• c) Sporotrichosis: Typically occurs after skin inoculation (e.g., from plants) and is not strongly associated with AIDS.

• d) Dermatophytosis: Superficial skin infections that affect immunocompetent and immunocompromised hosts alike but are not a hallmark of AIDS.

The ability to exist as yeast at 37°C (body temperature) and mold at 25°C (room temperature) is the defining characteristic of dimorphic fungi. This temperature-dependent transition is a key virulence trait that allows these fungi to adapt from the environment to the human host. Examples include:

• Histoplasma capsulatum

• Blastomyces dermatitidis

• Coccidioides immitis (though it forms spherules, not typical yeasts, at 37°C)

• Sporothrix schenckii

• Talaromyces marneffei

Why not the others?

• a) Dermatophytes: Grow only as molds at both temperatures (no yeast phase).

• c) Mucormycetes (e.g., Mucor, Rhizopus): Grow as molds at both temperatures (nonseptate hyphae).

• d) Dematiaceous fungi (e.g., Exophiala, Cladophialophora): Remain molds at both temperatures (pigmented hyphae).

The “spaghetti and meatball” appearance is a classic microscopic description for Malassezia furfur (a lipophilic yeast) in a potassium hydroxide (KOH) preparation of skin scrapings. This refers to:

• Short, curved hyphal forms (“spaghetti”)

• Clusters of round, thick-walled yeast cells (“meatballs”)

This pattern is pathognomonic for tinea versicolor (pityriasis versicolor), a superficial skin infection caused by Malassezia species.

Why not the others?

• a) Dermatophyte hyphae and spores: Appear as long, branching, septate hyphae and/or arthroconidia (e.g., in tinea corporis or capitis), not the “spaghetti and meatball” pattern.

• c) Candida pseudohyphae and blastoconidia: Show elongated pseudohyphae with budding yeast cells (blastoconidia), typically in thrush or vaginitis, but lack the distinct “meatball” clusters.

• d) Aspergillus hyphae and conidiophores: Display septate hyphae with acute-angle branching and specialized conidiophores (e.g., in aspergillosis), not the Malassezia-specific morphology.

The description of “sleeve” macroconidia with echinulations (rough or spiny walls) is characteristic of Microsporum canis.

• Macroconidia of M. canis are spindle-shaped, thick-walled, and typically have a rough, spiny surface (echinulations).

• They are often described as “sleeve-like” due to their elongated, tapered ends and numerous septa.

• This morphology helps differentiate M. canis from other dermatophytes.

Why not the others?

• b) Trichophyton rubrum: Produces pencil-shaped or cylindrical macroconidia with smooth walls (not echinulate) and is often rare or absent; microconidia are more common.

• c) Epidermophyton floccosum: Forms smooth-walled, club-shaped macroconidia that occur in clusters (not rough-walled).

• d) Trichophyton mentagrophytes: Produces round microconidia in clusters and spiral hyphae; its macroconidia (if present) are smooth-walled and cigar-shaped.

Tinea cruris is a fungal infection of the groin, inner thighs, and buttocks area. It is commonly known as jock itch due to its frequent occurrence in athletes or individuals who experience prolonged moisture and friction in these areas (e.g., from sweating or tight clothing). It is typically caused by dermatophytes like Trichophyton rubrum or Epidermophyton floccosum.

Why not the others?

• a) Athlete’s foot: Refers to tinea pedis (fungal infection of the feet).

• b) Ringworm of the scalp: Refers to tinea capitis.

• d) Nail infection: Refers to tinea unguium (onychomycosis).

The “blueberry muffin” baby presentation refers to a newborn with widespread purpuric lesions (blue-red nodules) that resemble blueberry muffins. This is not typically associated with fungal infections. Instead, it is classically linked to congenital infections such as:

• Rubella (German measles)

• Cytomegalovirus (CMV)

• Other TORCH infections (Toxoplasmosis, Syphilis, Varicella-Zoster, Parvovirus B19)

While severe congenital candidiasis can cause skin lesions in newborns, they usually appear as erythematous papules or pustules rather than the classic “blueberry muffin” purpuric pattern. The term is primarily reserved for the viral and bacterial infections mentioned above.

Why not the others?

• a) Candida albicans: Can cause congenital candidiasis with skin involvement, but the lesions are typically pustular or vesicular, not purpuric “blueberry muffin” nodules.

• b) Cryptococcus neoformans: Rarely causes congenital infection and does not present with this pattern.

• c) Histoplasma capsulatum: Congenital histoplasmosis is extremely rare and not associated with this presentation.

The hair perforation test is a laboratory method used to distinguish between two common dermatophytes:

• Trichophyton mentagrophytes is positive for hair perforation. It produces enzyme-like structures that create wedge-shaped perforations in hair shafts when tested in vitro.

• Trichophyton rubrum is negative for hair perforation and does not cause these erosions.

This test helps differentiate these species, which can otherwise appear similar in culture.

Why not the others?

• b) Microsporum canis and Microsporum audouinii: Differentiated by growth characteristics and conidia morphology, not the hair perforation test.

• c) Candida albicans and Candida tropicalis: Identified through germ tube tests, carbohydrate assimilation, or chromogenic agar.

• d) Aspergillus fumigatus and Aspergillus flavus: Distinguished by colony color, conidial head morphology, and temperature tolerance, not hair perforation.

The phenol oxidase test (using media like birdseed agar or caffeic acid agar) is a key diagnostic test for Cryptococcus neoformans. This test detects the enzyme phenol oxidase, which Cryptococcus neoformans produces. When the yeast is grown on birdseed agar, it utilizes compounds in the agar (such as caffeic acid) to produce melanin, resulting in characteristic brown to black pigmentation of the colonies. This is a defining feature that helps differentiate C. neoformans from other yeasts.

Why not the others?

• a) Candida albicans: Does not produce phenol oxidase and will not form brown pigment on birdseed agar; it is identified by germ tube tests or chromogenic agar.

• c) Histoplasma capsulatum: A dimorphic fungus that does not produce melanin via phenol oxidase on birdseed agar; it is identified by its conversion to yeast form at 37°C and specific antigens.

• d) Blastomyces dermatitidis: Also dimorphic and does not exhibit this test; it is identified by its broad-based budding yeast form in tissues.

While all the listed structures can be found in dermatophytes, the size, shape, wall texture, and arrangement of the macroconidia are the primary characteristics used to differentiate genera (e.g., Microsporum, Trichophyton, Epidermophyton) and species within the group. Microconidia also aid in identification but are often less distinctive.

• a) Septate hyphae: This is a common feature of most pathogenic fungi and is not specific to dermatophytes.

• b) Racquet hyphae: This is a specialized hyphal structure sometimes seen in dermatophyte cultures, but it is not consistently present or unique enough for definitive identification.

• d) Chlamydospores: These are survival structures produced by many fungi, including some dermatophytes, but they are not the key diagnostic feature.

The “black dot” tinea capitis is characterized by hair shafts that break off at the scalp surface, leaving behind dark, stubbly remnants that resemble black dots. This pattern is most commonly caused by Trichophyton tonsurans. This dermatophyte is an endothrix fungus, meaning it invades the inside of the hair shaft (hyphae and spores within the hair), leading to fragility and breakage just at the scalp level.

Why not the others?

• a) Microsporum canis: Causes hair infections but typically results in hair breaking off above the scalp (ectothrix infection) and may fluoresce green under Wood’s lamp. It does not produce the “black dot” appearance.

• c) Epidermophyton floccosum: Infects skin and nails but does not cause hair infections (tinea capitis).

• d) Microsporum audouinii: Can cause tinea capitis with ectothrix infection and may fluoresce, but it is less common and not associated with the “black dot” pattern.

Onychomycosis is the medical term for a fungal infection of the fingernails or toenails. It is commonly caused by dermatophytes (e.g., Trichophyton rubrum), but yeasts (e.g., Candida) and non-dermatophyte molds can also be involved. Symptoms include nail thickening, discoloration (yellow, white, or brown), brittleness, and debris under the nail.

Why not the others?

• a) Tinea corporis: Refers to fungal infection of the body (ringworm).

• c) Tinea pedis: Refers to fungal infection of the feet (athlete’s foot).

• d) Tinea barbae: Refers to fungal infection of the beard and facial hair.

Coccidioidomycosis, caused by the dimorphic fungi Coccidioides immitis and Coccidioides posadasii, is commonly known as “Valley Fever”. This name originates from its endemic occurrence in arid regions of the southwestern United States (e.g., San Joaquin Valley in California), as well as parts of Mexico and Central and South America. The infection is typically acquired by inhaling spores from contaminated soil.

Why not the others?

• b) Histoplasmosis: Often called “Ohio Valley disease” or “cave disease,” but not “Valley Fever.”

• c) Blastomycosis: Sometimes referred to as “Chicago disease” or “North American blastomycosis,” but not “Valley Fever.”

• d) Cryptococcosis: No widely recognized geographic nickname; it is associated with bird droppings (e.g., pigeon excreta).

Pneumocystis jirovecii (formerly P. carinii), the causative agent of pneumocystis pneumonia (PCP), is best visualized using the Gomori methenamine silver (GMS) stain. This stain specifically highlights the cell wall of the fungus, turning the cysts black or brown against a green background. The characteristic appearance includes cup-shaped or helmet-shaped cysts, which are diagnostic for PCP. GMS is highly sensitive and is considered a gold standard for detecting P. jirovecii in respiratory specimens (e.g., bronchoalveolar lavage or lung tissue).

Why not the others?

• a) Gram stain: P. jirovecii stains poorly and inconsistently with Gram stain, making it unreliable for diagnosis.

• b) KOH: Used for clearing keratin in skin/hair/nail samples for dermatophytes but is not effective for P. jirovecii in respiratory samples.

• d) India ink: Used to detect the capsule of Cryptococcus neoformans in CSF, not P. jirovecii.

The germ tube test is a rapid diagnostic test used to presumptively identify Candida albicans (and the closely related Candida dubliniensis). When incubated in human or animal serum at 37°C for 2–3 hours, C. albicans produces germ tubes (short, filamentous outgrowths without constrictions at their point of origin from the yeast cell). This is a key distinguishing feature, as most other Candida species do not form true germ tubes under these conditions.

Why not the others?

• b) Candida glabrata: Does not produce germ tubes; it forms small, budding yeast cells only.

• c) Candida tropicalis: May produce pseudohyphae but not true germ tubes (it is germ tube-negative).

• d) Candida krusei: Does not produce germ tubes; it forms elongated, cylindrical yeast cells and pseudohyphae.

Mucormycosis (caused by fungi in the order Mucorales, such as Rhizopus, Mucor, and Lichtheimia) is characterized by:

• Broad, non-septate (or sparsely septate) hyphae: These appear ribbon-like and irregular in width.

• Right-angle (90°) branching: This distinguishes them from other molds like Aspergillus, which have septate hyphae with acute-angle (45°) branching.

This morphology is a key diagnostic feature when examining tissue samples (e.g., via KOH preparation or histopathology) and is particularly associated with rhinocerebral mucormycosis in diabetic patients with ketoacidosis.

Why not the others?

• a) Septate hyphae with acute angles: Typical of Aspergillus species (e.g., in aspergillosis).

• c) Yeast cells only: Seen in infections caused by yeasts like Candida or Cryptococcus.

• d) Spherules: Characteristic of Coccidioides immitis in tissues.

Fusarium species are opportunistic molds that are particularly notorious for causing disseminated infections in severely neutropenic patients (e.g., those undergoing chemotherapy or with hematologic malignancies). These infections can present with:

• Fungemia (positive blood cultures, unlike many other molds).

• Skin lesions (ecthyma-like nodules).

• Multiorgan involvement.

The prognosis is poor due to the aggressiveness of the fungus and the underlying immunosuppression.

Why not the others?

• a) Nail infections (onychomycosis) in immunocompetent hosts: While Fusarium can cause onychomycosis, it is rare compared to dermatophytes, and its primary clinical significance lies in opportunistic systemic infections.

• c) Cutaneous infections only in tropical regions: Fusarium can cause localized skin infections (e.g., keratitis, mycetoma) but is not restricted to tropical regions and is more feared for disseminated disease.

• d) Pulmonary allergic disease (ABPA): Allergic bronchopulmonary aspergillosis (ABPA) is specifically caused by Aspergillus species, not Fusarium.

Ringworm of the scalp (tinea capitis) is primarily caused by dermatophytes, and Microsporum audouinii is one of the classic agents. This fungus:

• Invades hair shafts, leading to scaling, hair loss, and sometimes inflammation.

• Often fluoresces apple-green under a Wood’s lamp (ultraviolet light), aiding diagnosis.

• Is spread through direct contact with infected individuals or fomites (e.g., combs, hats).

Why not the others?

• a) Malassezia furfur: Causes tinea versicolor (pityriasis versicolor), a superficial skin infection, not scalp ringworm.

• c) Candida albicans: Causes oral thrush, vaginitis, or skin infections but is not a common cause of tinea capitis.

• d) Histoplasma capsulatum: A dimorphic fungus that causes systemic histoplasmosis (lung infection), not superficial scalp infections.

Coccidioides immitis (and Coccidioides posadasii) is a dimorphic fungus with a unique life cycle:

• In the environment (soil in arid regions), it grows as a mold that produces fragile, airborne arthroconidia (barrel-shaped asexual spores).

• When these conidia are inhaled into the lungs, they transform into characteristic spherules—large, round structures that contain endospores. This transformation defines its dimorphic nature and is key to its pathogenesis.

Why not the others?

• a) Histoplasma capsulatum: Inhaled microconidia transform into yeast forms in the lungs (not spherules).

• b) Blastomyces dermatitidis: Inhaled conidia convert to broad-based budding yeasts in the lungs.

• d) Sporothrix schenckii: Typically enters via skin inoculation (e.g., thorns) and transforms to yeast forms in tissue; not primarily inhaled as conidia causing pulmonary spherules.

The phenol oxidase test (often using caffeic acid or birdseed agar) is a key test to differentiate Cryptococcus neoformans (and C. gattii) from other yeasts. This test detects the enzyme phenol oxidase, which Cryptococcus species possess. When grown on media containing compounds like caffeic acid, C. neoformans produces melanin, resulting in brown to black pigmentation of the colonies. This is a highly specific characteristic.

Why not the others?

• a) Germ tube test: Used to identify Candida albicans (positive), but Cryptococcus is negative.

• b) Urease test: Cryptococcus is urease-positive, but other yeasts (e.g., Rhodotorula, some Trichosporon) can also be urease-positive, so it is not definitive alone.

• d) Lactophenol cotton blue mount: A general fungal stain for observing morphology but not specific for differentiation.

Rhizopus species (a genus of Zygomycetes or Mucormycetes) are characterized by the presence of rhizoids (root-like structures) that anchor the fungus to the substrate. Key identifying features of Rhizopus include:

• Sporangiophores (aerial hyphae) that arise in clusters from stolons (runner hyphae).

• Rhizoids located at the base of the sporangiophores, opposite the point where the stolon attaches.

• Large, spherical sporangia filled with sporangiospores.

This distinguishes Rhizopus from other mucormycetes (e.g., Mucor, which lacks rhizoids and has sporangiophores that arise randomly).

The other options are incorrect:

• b) Phialides: Flask-shaped structures that produce conidia in fungi like Aspergillus or Penicillium.

• c) Arthroconidia: Barrel-shaped asexual spores formed by fragmentation of hyphae (e.g., in Coccidioides or dermatophytes).

• d) Blastoconidia: Budding yeast cells (e.g., in Candida species).

• The germ tube test is a rapid test used to presumptively identify Candida albicans (and sometimes Candida dubliniensis).

• When incubated in serum at 37 °C for about 2–3 hours, C. albicans produces germ tubes (tube-like outgrowths without constriction at their origin).

• Other options:

  • Cryptococcus neoformans → Identified by India ink prep or urease test, not germ tube test.

  • Aspergillus fumigatus → Identified by conidial morphology, not germ tube.

  • Histoplasma capsulatum → Diagnosed by dimorphic conversion or histopathology, not germ tube.

The “spaghetti and meatball” appearance is a classic microscopic description for Malassezia furfur (a lipophilic yeast) in a potassium hydroxide (KOH) preparation of skin scrapings. This refers to:

• Short, curved hyphal forms (“spaghetti”)

• Clusters of round, thick-walled yeast cells (“meatballs”)

This pattern is pathognomonic for tinea versicolor (pityriasis versicolor), a superficial skin infection caused by Malassezia species.

Why not the others?

• a) Dermatophytes: Appear as long, branching, septate hyphae and/or arthroconidia (e.g., in tinea corporis or capitis), not the “spaghetti and meatball” pattern.

• b) Candida albicans: Shows budding yeast cells, pseudohyphae, or true hyphae (e.g., in thrush or vaginitis), but lacks the distinct “meatball” clusters.

• d) Aspergillus fumigatus: Displays septate hyphae with acute-angle branching and specialized conidiophores (e.g., in aspergillosis), not the Malassezia-specific morphology.

The KOH (potassium hydroxide) preparation is the primary test used to differentiate fungal infections (e.g., dermatophytosis, tinea versicolor) from non-fungal skin conditions like psoriasis, eczema, or lichen planus.

• How it works: KOH dissolves keratin and non-fungal cellular material in skin, hair, or nail scrapings, allowing clear visualization of fungal elements (hyphae, spores, or yeast cells) under a microscope.

• Purpose: If fungal structures are seen, it confirms a fungal infection; if absent, non-fungal causes (e.g., psoriasis) are more likely.

Why not the others?

• a) India ink: Used to detect the capsule of Cryptococcus neoformans in CSF, not skin fungi.

• b) Giemsa stain: Used for blood parasites (e.g., malaria) or certain intracellular pathogens, not routine fungal diagnosis.

• d) Silver stain (e.g., GMS): Highly sensitive for fungi in tissue biopsies but is complex and not used as a first-line test for superficial skin scrapings.