Internal autoinfection occurs when larvae of Strongyloides stercoralis mature inside the host’s intestine and then penetrate the intestinal wall (or perianal skin) without leaving the body.

• This allows the parasite to maintain infection for years without new exposure from the environment.

• It can lead to hyperinfection syndrome, especially in immunocompromised individuals.

Why not the others?

• Ascaris lumbricoides → Requires external soil maturation of eggs.

• Necator americanus (hookworm) → Needs external skin penetration by larvae from soil.

• Trichuris trichiura (whipworm) → No autoinfection; requires ingestion of mature eggs from the environment.

• In permanent stained smears (like trichrome stain), Entamoeba histolytica trophozoites can be identified by the presence of ingested red blood cells (RBCs) inside their cytoplasm — a key feature that helps differentiate it from nonpathogenic amoebae like Entamoeba dispar.

• b) Large karyosome with no chromatin → not characteristic of E. histolytica.

• c) Flagella → E. histolytica is an amoeba, no flagella.

• d) Polar filaments → feature of Trichomonas vaginalis trophozoites, not amoebae.

• Thick blood smears concentrate a larger volume of blood into a smaller area by lysing RBCs during staining.

• This makes it easier to detect even low levels of parasitemia, since more parasites are present in the field of view.

• However, species identification is more difficult on thick smears — that’s why thin smears are used for precise species differentiation.

Why not the others?

• a) Enhance parasite staining → Staining is not inherently “enhanced” by thickness; it’s the concentration that matters.

• c) Remove platelet artifacts → Not the main purpose.

• d) Differentiate species easily → Thick smears are worse for species ID; thin smears are better for that.

The Baermann technique uses warm water to stimulate larvae to actively migrate out of the stool sample into the surrounding fluid, concentrating them for detection. It’s especially useful for detecting Strongyloides stercoralis larvae.

• a) Helminth eggs → detected by sedimentation or flotation methods, not Baermann.

• c) Coccidian oocysts → detected by acid-fast staining.

• d) Microsporidia spores → detected by special stains or PCR.

Chagas disease (American trypanosomiasis) is caused by Trypanosoma cruzi and transmitted primarily by:

• Reduviid bugs (Triatominae), also called “kissing bugs,” which defecate near the bite wound, allowing the parasite (in feces) to enter the host’s bloodstream.

Why Not Other Options?

• a) Anopheles mosquito: Transmits Plasmodium (malaria), not T. cruzi.

• b) Ixodes tick: Transmits Lyme disease (Borrelia burgdorferi) and babesiosis.

• d) Aedes mosquito: Transmits dengue, Zika, and yellow fever viruses.

• Motile trophozoites of protozoa (e.g., Entamoeba histolytica, Giardia lamblia) rapidly lose motility and morphological detail after passing in stool.

• Fresh diarrheal specimens should be examined within 15 minutes to detect motility.

• Why not the others?

  • 1 hour / 3 hours / 6 hours → too late; trophozoites will have degenerated and lost motility, making detection unreliable.

• Cryptosporidium parvum oocysts appear red/pink on modified acid-fast stain against a blue background.

• Why not the others?

  • Gram stain → not useful; oocysts don’t stain well with Gram.

  • Giemsa stain → used for Plasmodium, Trypanosoma, Leishmania, and some intracellular bacteria.

  • Calcofluor white → used for fungi (stains chitin in cell walls).

The formalin-ethyl acetate concentration method is designed to separate parasites (eggs, cysts, larvae) from fecal debris by sedimentation. This reduces debris and concentrates the parasites in the sediment, improving detection under the microscope.

• a) Killing motile forms → killing can happen but it’s not the purpose or a benefit.

• c) Concentrating bacterial flora → this is not desired and not the aim.

• d) Enhancing color contrast → staining does that, not concentration method.

• Trichomonas vaginalis is a flagellated protozoan that causes trichomoniasis, a common sexually transmitted infection.

• It is transmitted primarily through vaginal, penile, or urethral contact during sex.

• The organism exists only as a trophozoite (no cyst stage), so survival outside the host is limited — direct person-to-person transmission is essential.

Why not the others?

• Fecal-oral route → Common for intestinal protozoa (e.g., Giardia, Entamoeba), not T. vaginalis.

• Inhalation of cysts → Applies to some free-living amoebae or fungi, not T. vaginalis.

• Skin penetration → Seen in parasites like hookworms and Strongyloides, not T. vaginalis.

Hymenolepis nana (dwarf tapeworm) infection is diagnosed by identifying its characteristic eggs in stool samples. Here’s why:

1. Egg Morphology:

   • Size: 30–50 µm in diameter.

   • Appearance: Transparent, spherical, with a thin outer membrane and six hooklets inside the oncosphere.

   • Polar filaments: Visible between the oncosphere and egg shell, a key diagnostic feature .

2. Life Cycle & Transmission:

   • Eggs are released into the intestinal lumen by adult worms and excreted in feces.

   • Direct fecal-oral transmission (common in children) or ingestion of infected arthropods (fleas/beetles) can occur .

Why Not Other Options?

• a) Blood: H. nana does not circulate in blood (unlike Schistosoma or microfilariae).

• b) Sputum: Irrelevant; pulmonary involvement is rare and not diagnostic.

• d) Urine: Eggs are not excreted in urine (unlike Schistosoma haematobium).

Primary amoebic meningoencephalitis (PAM) is a rare but almost invariably fatal brain infection caused by the free-living amoeba Naegleria fowleri. This organism thrives in warm freshwater environments (e.g., lakes, hot springs, poorly chlorinated pools) and infects humans when contaminated water enters the nasal passages, allowing the amoeba to migrate to the brain via the olfactory nerve1311.

Why Not the Other Options?

• a) Entamoeba coli → A non-pathogenic intestinal amoeba, unrelated to CNS infections2.

• b) Dientamoeba fragilis → A protozoan causing gastrointestinal symptoms, not encephalitis2.

• c) Endolimax nana → Another non-pathogenic intestinal amoeba, harmless to humans

Cyclospora cayetanensis oocysts are uniquely identified by their blue autofluorescence under ultraviolet (UV) microscopy, which is a key diagnostic feature. Here’s why:

• UV Autofluorescence: When viewed under a fluorescence microscope (365 nm wavelength), Cyclospora oocysts emit a distinctive blue-green glow, aiding rapid screening .

• Size & Morphology:

  • Oocysts are 8–10 µm in diameter (smaller than Cryptosporidium at 4–6 µm but larger than Microsporidia).

  • They are round and non-refractile, with undifferentiated contents in fresh stool .

Why Not Other Options?

• a) Acid-fast staining: Cyclospora is variably acid-fast (weaker than Cryptosporidium), but this is less specific than autofluorescence .

• c) Bipolar plugs: Seen in Trichuris trichiura (whipworm) eggs, not Cyclospora.

• d) Large size (10–15 µm): Incorrect; Cyclospora oocysts are smaller (8–10 µm). Isospora belli reaches 20–30 µm

• Trichomonas vaginalis trophozoites are fragile and motile; to maximize recovery, vaginal swabs should be inoculated into culture medium promptly (within 1 hour) to allow trophozoite growth.

• a) Fixed immediately in PVA → good for microscopy but kills trophozoites, no culture possible.

• c) Frozen at -20°C → kills trophozoites, not recommended.

• d) Preserved in formalin → kills trophozoites, unsuitable for culture.

• Polyvinyl alcohol (PVA) contains mercuric chloride and other components that interfere with acid-fast staining, making it unsuitable for detecting Cryptosporidium oocysts.

• Why not the others?

  • Formalin, MIF, and SAF → all are compatible with acid-fast stains and can be used for Cryptosporidium detection.

1. Why not room temperature (a)?

   • Stool left at room temperature can degrade quickly, allowing bacterial overgrowth or parasite disintegration, which may lead to false-negative results.

2. Why not formalin (b)?

   • While formalin (usually 10% buffered formalin) is an excellent preservative for stool samples, it is typically used when long-term storage or transport is needed. If the lab plans to process the sample the next day, refrigeration is sufficient.

3. Why not freeze at -20°C (c)?

   • Freezing can destroy parasite morphology (especially trophozoites or delicate eggs), making identification difficult under the microscope.

4. Why refrigerate at 4°C (d)?

   • Refrigeration slows bacterial growth and preserves parasite structures (ova, cysts, larvae) for at least 24–48 hours.

   • This is the standard recommendation by clinical microbiology guidelines (e.g., CDC, CLSI) for short-term delays in O&P processing.

• Primary amebic meningoencephalitis (PAM) is caused by Naegleria fowleri.

• Infection occurs when contaminated freshwater enters the nose (e.g., swimming, diving), allowing amoebae to migrate along the olfactory nerve to the brain.

• Why not the others?

  • Contaminated food → not a transmission route for PAM.

  • Sexual contact → not involved in its transmission.

  • Mosquito bites → not relevant; PAM is not vector-borne.

• Proper mixing of the stool sample with formalin and ethyl acetate ensures thorough emulsification and helps separate parasites from debris during centrifugation, leading to accurate concentration and recovery.

• a) High-speed centrifugation → standard speed (~500g) is sufficient; too high can damage parasites.

• b) Addition of iodine → not part of this concentration step; iodine is for temporary staining.

• d) Freezing → not done before processing as it can damage parasites.

NNN medium (Novy-MacNeal-Nicolle) is the classic and gold standard for culturing Leishmania species. Key features:

• Composition:

  • Agar base with defibrinated rabbit blood (provides nutrients).

  • Locke’s solution (maintains pH/osmolarity).

• Purpose: Supports growth of promastigotes (motile extracellular form).

• Use: Diagnosing cutaneous/visceral leishmaniasis from clinical samples (e.g., bone marrow, skin biopsies).

Why Not Other Options?

• a) Blood agar: Used for bacteria (e.g., Streptococcus) and some fungi, not Leishmania.

• b) Sabouraud dextrose agar: Primarily for fungi (e.g., Candida); lacks nutrients for Leishmania.

• d) Chocolate agar: Enriched for fastidious bacteria (e.g., Haemophilus), not protozoa.

The “cellophane tape” method (also called the Scotch tape test) is used to collect eggs laid by Enterobius vermicularis (pinworm) around the perianal area, which are often missed in stool samples.

• a) Giardia lamblia cysts → detected in stool samples, not tape test.

• c) Strongyloides larvae → detected by stool examination or Baermann method.

• d) Ascaris lumbricoides eggs → detected in stool, not tape test.

• Giardia lamblia trophozoites are pear-shaped (or teardrop-shaped) protozoa with two nuclei that look like “eyes” and four pairs of flagella.

• In fresh wet mounts, they show falling-leaf motility — a slow, tumbling motion.

• In a trichrome-stained smear, the shape and internal structures (two nuclei, median bodies) are visible, but motility is not seen — however, the pear shape is still the key identification feature.

Why not the others?

• Ingested RBCs → Characteristic of Entamoeba histolytica trophozoites.

• Four nuclei with peripheral chromatin → Seen in E. histolytica/E. coli cysts, not Giardia trophozoites.

• Cigar-shaped morphology → Describes Trichuris trichiura eggs, not protozoan trophozoites.

Humans primarily acquire Toxoplasma gondii infection through:

1. Consumption of undercooked or raw meat (especially pork, lamb, or venison) containing tissue cysts with bradyzoites.

2. Ingestion of oocysts from contaminated food, water, or soil (e.g., via unwashed vegetables or contact with cat feces).

Why Not Other Options?

• a) Mosquito bites: T. gondii is not transmitted by arthropod vectors.

• c) Inhalation of spores: Toxoplasma spreads via ingestion, not inhalation (though rare lab-acquired cases via aerosols have been reported).

• d) Skin contact with soil: Intact skin is a barrier; infection requires oral ingestion of oocysts.

• The zinc sulfate flotation technique uses a solution with higher specific gravity, causing lighter parasite stages like protozoan cysts and some helminth eggs to float to the surface for easier collection and identification.

• It’s especially good for detecting protozoan cysts like Giardia and Entamoeba.

Other options:

• b) Detect motile trophozoites → trophozoites require wet mounts and fresh samples.

• c) Preserve morphology of larvae → flotation isn’t ideal for larvae; sedimentation methods are better.

• d) Identify microsporidia → special stains are used for microsporidia, not flotation.

Isospora belli (now reclassified as Cystoisospora belli) oocysts in stool are identified by their distinctive ellipsoidal shape and internal structure containing two sporocysts, each with four sporozoites. Key diagnostic features include:

• Size: 20–33 µm long and 10–19 µm wide, making them larger than Cryptosporidium (4–6 µm) and Cyclospora (8–10 µm) .

• Morphology:

  • Immature oocysts contain one sporoblast (spherical mass of protoplasm) when excreted .

  • Mature oocysts develop two sporocysts (each with four sporozoites) after 1–4 days in the environment .

• Staining:

  • Acid-fast positivity (b): Oocysts stain variably with modified Ziehl-Neelsen or Kinyoun stains but are less consistently acid-fast than Cryptosporidium .

  • Autofluorescence (d): Cystoisospora oocysts autofluoresce under UV microscopy, but this is less emphasized than for Cyclospora .

Why Not Other Options?

• b) Acid-fast positivity: Supports diagnosis but is not unique (shared with Cryptosporidium).

• c) Small size (4–5 µm): Incorrect; C. belli oocysts are much larger (20–33 µm) .

• d) Blue autofluorescence: Reported but not pathognomonic (also seen in Cyclospora)

Guinea worm disease, caused by Dracunculus medinensis, is uniquely diagnosed by:

• Emerging adult female worm: The worm creates a painful blister (usually on the legs/feet) and slowly emerges over weeks to release larvae when submerged in water.

• Diagnostic method: The worm is identified visually as it exits the skin (often wrapped around a stick during traditional extraction).

Why Not Other Options?

• b) River blindness (Onchocerciasis): Diagnosed via skin snips for microfilariae or nodule biopsy for adult Onchocerca volvulus.

• c) Elephantiasis (Lymphatic filariasis): Diagnosed by blood smears (microfilariae of Wuchereria bancrofti/Brugia spp.) or antigen tests.

• d) Cutaneous larva migrans (CLM): Diagnosed clinically (serpiginous tracks from Ancylostoma braziliense larvae); no adult worms are involved.

Nocturnal periodicity refers to the phenomenon where microfilariae circulate in the peripheral blood primarily at night (peaking between 10 PM and 2 AM), which is a hallmark of:

• Wuchereria bancrofti (and Brugia malayi), the causative agents of lymphatic filariasis (elephantiasis).

Key Points:

1. Adaptation to Mosquito Vectors:

   • The microfilariae’s nocturnal surge aligns with the night-biting habits of their mosquito vectors (Culex, Anopheles, Aedes).

   • In the Pacific variant (e.g., Fiji), W. bancrofti exhibits subperiodic (daytime) circulation, matching Aedes polynesiensis biting patterns .

2. Why Not Other Options?

   • a) Loa loa: Shows diurnal periodicity (peak microfilariae levels during daytime), coinciding with the biting habits of Chrysops deer flies .

   • b) Onchocerca volvulus: Microfilariae reside in skin nodules, not blood, and lack periodicity.

   • d) Mansonella perstans: Microfilariae circulate non-periodically in blood .

Schistosoma haematobium is the causative agent of urinary schistosomiasis, and its eggs are primarily detected in:

• Urine sediment (especially in terminal urine or urine collected around midday, when egg shedding peaks).

• The eggs are characteristically oval with a terminal spine, distinguishing them from other Schistosoma species.

Why Not Other Options?

• a) Stool: Eggs of S. mansoni (lateral spine) and S. japonicum (smaller, rounded) are found in stool, but S. haematobium rarely appears there.

• b) Sputum: Not relevant; pulmonary schistosomiasis may occur, but eggs are not typically found in sputum.

• d) Blood smears: Schistosome eggs are not detected in peripheral blood smears (unlike malaria or filariae).

Wuchereria bancrofti microfilariae exhibit nocturnal periodicity, meaning they appear in peripheral blood mostly at night (typically between 10 PM and 2 AM). Collecting blood during this time increases the chance of detecting microfilariae.

• a) Morning and b) Midday → low microfilariae levels, leading to false negatives.

• d) Anytime → not ideal due to periodicity.

The Scotch tape (cellophane tape) test is the gold standard for diagnosing pinworm infection (Enterobius vermicularis). Here’s why:

• Method: Transparent tape is pressed against the perianal region (best done early morning before bathing) to collect eggs or adult worms.

• Sensitivity: Pinworms lay eggs on perianal folds, not in stool, making stool samples (a) unreliable.

• Microscopy: Tape is examined under a microscope for characteristic flattened, asymmetrical eggs.

Why Not Other Options?

• a) Stool concentrate: Less effective because pinworm eggs are rarely found in stool.

• b) Duodenal aspirate: Used for parasites like Giardia or Strongyloides, not pinworms.

• d) Urine sediment: Irrelevant for Enterobius diagnosis.

• PVA fixative preserves protozoan trophozoites exceptionally well, maintaining their morphology for permanent stained smears (e.g., trichrome stain).

• Why not the others?

  • 10% formalin → good for cysts and helminth eggs/larvae, but not for delicate trophozoites.

  • SAF (Sodium acetate–acetic acid–formalin) → preserves cysts and trophozoites but not as well as PVA for stained smears.

  • Refrigeration at 4°C → slows degradation but doesn’t preserve morphology long-term for microscopy.

• Giardia lamblia trophozoites are fragile and lose motility quickly once the stool leaves the body — especially in liquid or watery stool.

• If examination is delayed (more than ~30 minutes), trophozoites may disintegrate, causing false negatives.

• That’s why immediate wet mount examination is essential for fresh diarrheal samples.

Other options:

• a) Multiple samples → actually increases detection rate.

• b) SAF fixative → preserves cysts, so not an error.

• d) Trichrome stain → good for detecting cysts and trophozoites (from preserved stool), not a cause of false negatives by itself.

• PVA (Polyvinyl alcohol) fixative preserves parasite morphology well for permanent stains but interferes with antigen-antibody reactions in immunoassays, inhibiting antibody binding.

• This makes it unsuitable for antigen detection assays like ELISA or immunofluorescence.

Other options:

• a) Contains mercury → true, but this is a safety concern, not why it’s bad for antigen assays.

• c) Causes cyst distortion → not the main issue here.

• d) Prevents concentration → concentration is a separate step, unrelated to fixative choice for antigen detection.

• In trichrome-stained smears, helminth eggs are recognized by their characteristic internal structures (e.g., operculum, polar plugs, larval form) and their size and shape.

• Why not the others?

  • Degree of motility → not applicable; eggs are non-motile.

  • Iodine uptake → used in wet mounts for cysts, not in trichrome staining.

  • Antibody detection → a serologic method, unrelated to microscopy.

ollen grains can resemble ova or cysts in stool specimens due to their similar size, shape, and sometimes their layered appearance. They are common environmental contaminants that may accidentally enter the sample during collection or processing.

• a) Meat fibers – Usually appear as irregular, fibrous structures and are not typically confused with parasites.

• b) Degenerated cells – These are amorphous and lack the distinct morphology of ova or cysts.

• c) Dried chemical crystals – Have geometric shapes (e.g., rhomboid, needle-like) and are not biological in origin.

The most reliable feature to distinguish Ancylostoma duodenale from Necator americanus is the structure of the buccal capsule (mouthparts) in adult worms:

• Ancylostoma duodenale:

  • Has 2 pairs of ventral teeth (sharp, cutting plates).

  • Larger in size (adults ~10–13 mm).

• Necator americanus:

  • Has 1 pair of dorsal cutting plates (semilunar ridges).

  • Smaller in size (adults ~7–9 mm).

Why Not Other Options?

• b) Egg size: Eggs of both species are morphologically identical (oval, thin-shelled, ~60×40 µm) and cannot be differentiated microscopically.

• c) Rhabditiform larvae shape: Larvae of both species are similar (though subtle differences exist, they are not diagnostic).

• d) Geographic distribution: Overlapping (both are global; N. americanus dominates in the Americas, A. duodenale in Mediterranean/Asia, but this is not a biological feature).

• SAF (Sodium acetate–acetic acid–formalin) fixative is versatile — it preserves protozoan cysts and helminth eggs/larvae well, and can be used for both:

  1. Permanent stained smears (e.g., trichrome stain)

  2. Concentration techniques (e.g., formalin-ethyl acetate sedimentation)

Other options:

• 10% formalin → good for concentration, but not ideal for permanent stained smears.

• PVA (Polyvinyl alcohol) fixative → good for permanent smears but contains mercury, not ideal for concentration.

• Lugol’s iodine → used as a temporary stain for wet mounts, not a fixative.

In thick blood films, the primary purpose is to increase sensitivity for detecting blood parasites (such as malaria, filaria, or trypanosomes). Here’s why:

1. RBCs are lysed during staining

   • Unlike thin films (where RBCs remain intact for morphology), thick films involve hemolysis (lysis of RBCs) during the staining process (e.g., Giemsa or Wright stain).

   • This releases parasites (e.g., malaria trophozoites or schizonts) from RBCs, allowing them to concentrate on the slide for easier detection .

2. Why other options are incorrect:

   • b) Fixed with methanol before staining → Methanol fixation is used in thin films to preserve RBC and parasite morphology, but it would prevent lysis in thick films.

   • c) Left intact for morphology → This describes thin films, where RBC integrity is maintained to assess parasite stages and species.

   • d) Used to identify malaria species → While thick films detect parasites, species identification is more reliable in thin films due to preserved RBC and parasite morphology .

3. Clinical relevance:

   • Thick films are 10–30 times more sensitive for parasite detection but require proper staining (lysis of RBCs is crucial).

   • Thin films are used for species confirmation after a positive thick film .

Polyvinyl alcohol (PVA) is a fixative and adhesive used in stool specimen preparation for permanent stained smears (e.g., trichrome stain). Its primary role is to help stool material stick firmly to the glass slide, preventing it from washing off during the multiple rinsing and staining steps.

• Why not the others?

  • a) Concentrates eggs → Concentration of eggs is done using flotation or sedimentation techniques, not PVA.

  • b) Dissolves artifacts → PVA doesn’t dissolve debris; it preserves morphology.

  • d) Enhances stain penetration → That’s the role of certain chemicals in staining solutions, not PVA itself.

1. Refrigeration reduces larval motility and recovery rates

   • Strongyloides stercoralis larvae are sensitive to cold temperatures, which can impair their movement and viability. Studies indicate that refrigeration significantly decreases the detection rate of larvae compared to fresh stool processing.

2. Fresh stool or short-term formalin exposure is preferred

   • The formalin-ether concentration technique (FECT) works best with fresh stool or short-term (not prolonged) formalin exposure, as prolonged formalin fixation (e.g., 10-min exposure) reduces larval recovery.

   • Direct wet mount within 1 hour is also effective because it captures live, motile larvae before degradation occurs.

3. Baermann concentration relies on larval motility

   • The Baermann technique depends on live larvae migrating out of stool into water. Refrigeration immobilizes larvae, making this method less effective if samples are not processed immediately.

4. Evidence from comparative studies

   • A study found that agar plate culture (APC), which requires live larvae, detected 95% of Strongyloides cases when using fresh stool, but sensitivity dropped significantly with refrigerated samples

Because many intestinal parasites (like Giardia, Entamoeba, and others) are shed intermittently, collecting 3 stool specimens on separate days significantly increases the chance of detecting the parasite. This is the standard recommendation for O&P (ova and parasite) exams, especially when symptoms like intermittent diarrhea occur.

• 1 or 2 specimens may miss parasites due to intermittent shedding.

• 5 specimens is more than usually recommended and often impractical.

The described findings—banana-shaped gametocytes, multiple ring forms per RBC, and infection of all RBC stages—are classic hallmarks of Plasmodium falciparum infection, the most virulent malaria parasite. Here’s why:

1. Banana-shaped gametocytes:

   • Unique to P. falciparum, these crescent- or banana-shaped gametocytes are a diagnostic feature not seen in other Plasmodium species (which have round/oval gametocytes) .

2. Multiple ring forms per RBC:

   • P. falciparum often exhibits multiple ring forms (≥2) within a single RBC, a rare feature in other species .

3. Infection of all RBC stages:

   • Unlike P. vivax and P. ovale (which preferentially infect reticulocytes) or P. malariae (which infects older RBCs), P. falciparum can invade RBCs of all ages, leading to high parasitemia and severe disease .

Why Not the Others?

• b) P. malariae: Shows “band-form” trophozoites and rosette schizonts; gametocytes are round .

• c) P. ovale: Infected RBCs are enlarged and oval with Schüffner’s dots; gametocytes are round .

• d) P. vivax: Similar to P. ovale but with ameboid trophozoites; gametocytes are round and fill the RBC

The unfertilized egg of Ascaris lumbricoides is recognized by its bipolar plugs (mucoid prominences at both ends) and other key features:

• Appearance:

  • Oval shape with a thick, mamillated (warty) outer shell (due to albuminous coating).

  • Unfertilized eggs are longer (~90×45 µm) and contain a disorganized mass of granules.

  • Fertilized eggs are rounder (~60×45 µm), with a smooth shell and a central developing embryo.

• Diagnostic significance: The bipolar plugs are pathognomonic for A. lumbricoides eggs.

Why Not Other Options?

• b) Barrel shape: Characteristic of Trichuris trichiura (whipworm) eggs, not Ascaris.

• c) Thin transparent shell: Seen in Enterobius vermicularis (pinworm) eggs, which are flattened on one side.

• d) Embryonated appearance: Ascaris eggs are not embryonated when passed in stool (unlike Ancylostoma or Strongyloides eggs, which may hatch rapidly).

Skin snips are a diagnostic method used to detect Onchocerca volvulus, the causative agent of river blindness (onchocerciasis). This technique involves taking small superficial skin samples, which are then examined microscopically for the presence of microfilariae.

Why not the others?

• Wuchereria bancrofti (a) and Brugia malayi (d) are typically diagnosed via blood smears (nocturnal periodicity for W. bancrofti).

• Loa loa (b) is also detected through blood smears (diurnal periodicity) or sometimes by visualizing the adult worm in the eye (“eye worm”).

• Entamoeba histolytica and Entamoeba hartmanni are morphologically similar, but they differ mainly in size.

• Trophozoites of E. histolytica are larger (usually 12–60 µm) compared to E. hartmanni (usually ≤12 µm).

• Both have similar nuclear features — a small, central karyosome and fine, even peripheral chromatin — so nuclear appearance alone cannot reliably distinguish them.

Why not the others?

• a) Number of nuclei in cysts → Both can have up to 4 nuclei in mature cysts.

• c) Appearance of karyosome → Both have a small, central karyosome.

• d) Presence of peripheral chromatin → Both have fine, evenly distributed peripheral chromatin.

• Visceral larva migrans (VLM) is most often caused by Toxocara canis (dog roundworm) or Toxocara cati (cat roundworm).

• Humans are accidental hosts — larvae migrate through organs like the liver, lungs, and eyes but do not mature.

• Why not the others?

  • Ancylostoma braziliense → causes cutaneous larva migrans (skin).

  • Strongyloides stercoralis → causes strongyloidiasis, not VLM.

  • Ascaris suum → pig roundworm; can infect humans but typically causes a disease similar to human Ascaris, not classic VLM

Scabies, caused by the mite Sarcoptes scabiei var. hominis, is diagnosed through microscopic examination of skin scrapings taken from burrows or papules. Here’s why:

1. Diagnostic Method:

   • A scalpel or curette is used to scrape the superficial epidermis (especially from interdigital spaces, wrists, or elbows) to extract mites, eggs, or fecal pellets (scybala).

   • The sample is placed on a slide with mineral oil or KOH and examined under a microscope.

2. Key Findings:

   • Adult mites: Oval, 0.2–0.5 mm, with four pairs of legs.

   • Eggs: Oval (~0.1 mm) and often found in short, linear burrows.

   • Fecal pellets: Brownish, round scybala confirm infection even if mites are absent .

Why Not Other Options?

• a) Blood smears: Irrelevant; scabies mites live exclusively in the epidermis.

• c) Stool samples: Used for intestinal parasites, not ectoparasites.

• d) Hair follicles: Scabies mites do not infect hair (unlike Demodex mites).

In the formalin-ethyl acetate sedimentation technique (a fecal concentration method used in parasitology), the steps are as follows:

1. The stool sample is mixed with formalin (to preserve parasites).

2. Ethyl acetate is added, and the mixture is centrifuged.

3. After centrifugation, the solution separates into distinct layers:

   • Top layer: Ethyl acetate (used to extract debris and fat).

   • Middle layer: Fecal debris.

   • Bottom layer: Formalin with concentrated parasites (this is the layer examined microscopically).

Thus, the top layer after centrifugation is ethyl acetate.

Enterobius vermicularis (pinworm) is most likely to be missed if a stool sample is collected only once. This is because Enterobius eggs are not typically shed in the stool. Instead, the female worm lays eggs around the perianal region, especially at night. Diagnosis is usually made using the tape test, where adhesive tape is applied to the perianal area to collect eggs, rather than through stool examination.

For other parasites like Giardia lamblia, Strongyloides stercoralis, and Ascaris lumbricoides, stool examination is more effective, although multiple samples may still be needed to increase sensitivity.

The Kato-Katz thick smear method is a stool microscopy technique designed to detect and quantify eggs of intestinal helminths — particularly Schistosoma, Ascaris, Trichuris, and hookworms.

• a) Blood protozoa → detected via blood smear, not Kato-Katz.

• c) Coccidian oocysts (Cryptosporidium, Cyclospora) → diagnosed with modified acid-fast stain.

• d) Flagellates in urine (Trichomonas vaginalis) → diagnosed with wet mount of urine or vaginal swab, not Kato-Katz.

Fasciola hepatica (the sheep liver fluke) requires a freshwater snail as its intermediate host to complete its life cycle. Here’s how it works:

1. Life Cycle Stages:

   • Eggs are passed in the feces of definitive hosts (humans, sheep, cattle).

   • In water, eggs hatch into miracidia, which infect snails (e.g., Galba truncatula or Lymnaea species).

   • Inside the snail, miracidia develop into sporocysts → rediae → cercariae.

   • Cercariae leave the snail and encyst as metacercariae on aquatic plants (e.g., watercress).

2. Human Infection:

   • Occurs by ingesting raw aquatic vegetation (e.g., watercress) contaminated with metacercariae.

   • Metacercariae excyst in the duodenum, migrate through the liver, and mature into adult flukes in bile ducts .

Why Not Other Options?

• b) Fish: Not involved; fish are intermediate hosts for Opisthorchis/Clonorchis.

• c) Copepod: Intermediate hosts for Diphyllobothrium latum (fish tapeworm).

• d) Ant: Non-relevant; ants are hosts for Dicrocoelium dendriticum (lancet fluke).

• In thick blood smears, fixation is omitted so that red blood cells (RBCs) lyse during staining, which concentrates the parasites on the slide and increases sensitivity.

• Fixing would preserve RBCs, but thick smears rely on their lysis to see parasites clearly.

Other options:

• a) Parasites are alcohol resistant → not relevant here.

• c) It preserves RBC morphology → this is true for thin smears, not thick smears.

• d) It enhances antibody binding → not related to thick smear preparation.

Balantidium coli is the only ciliated protozoan that causes significant human disease (balantidiasis). Key features:

• Morphology:

  • Large (50–200 µm), motile trophozoite with cilia covering its surface.

  • Contains a macronucleus (kidney-shaped) and micronucleus.

• Pathology: Causes colonic ulcers (similar to amebiasis) via invasion of intestinal mucosa.

• Transmission: Fecal-oral route (ingestion of cysts from contaminated water/food).

Why Not Other Options?

• a) Amoeba: Entamoeba histolytica is the major pathogenic amoeba.

• b) Flagellate: Includes Giardia, Trichomonas, and Trypanosoma.

• d) Sporozoan: Includes Plasmodium (malaria), Cryptosporidium, and Toxoplasma.

The modified acid-fast stain is used to detect coccidian parasites in stool, including:

• Cyclospora cayetanensis

• Cryptosporidium parvum

• Cystoisospora belli

Other options:

• Giardia lamblia cysts → detected by wet mount or trichrome stain, not acid-fast.

• Trichomonas vaginalis trophozoites → seen in wet mounts of vaginal/urine samples.

• Entamoeba histolytica cysts → diagnosed with wet mount or permanent stains (e.g., trichrome).

• Echinococcus granulosus is a small tapeworm whose larval stage causes hydatid disease (cystic echinococcosis) in humans.

• Humans are accidental intermediate hosts, acquiring infection by ingesting eggs from dog feces.

• The larvae migrate via the bloodstream, most often to the liver (but also lungs and other organs), where they develop into fluid-filled hydatid cysts.

Why not the others?

• Taenia saginata → Beef tapeworm; no hydatid cysts.

• Diphyllobothrium latum → Fish tapeworm; causes B₁₂ deficiency, not cystic lesions.

• Hymenolepis nana → Dwarf tapeworm; no hydatid cyst formation.

• Modified trichrome stain (Weber’s stain) is the preferred method to visualize microsporidia spores in stool specimens. It highlights the spores clearly with distinctive internal structures.

• Gram stain → not sensitive for microsporidia spores.

• Giemsa stain → used for blood parasites, not ideal for microsporidia.

• Acid-fast stain → useful for coccidian parasites like Cryptosporidium, but not for microsporidia.

Paragonimus westermani eggs are most reliably detected in sputum due to the parasite’s life cycle and pathology:

1. Life Cycle: Adult flukes reside in the lungs, where they lay eggs that are coughed up via the bronchial tree. Eggs are either expectorated in sputum or swallowed and later excreted in stool .

2. Diagnostic Sensitivity:

   • Sputum examination is the gold standard, as eggs are directly expelled from pulmonary lesions. Concentration techniques (e.g., sedimentation) improve detection.

   • Stool (a) may contain eggs from swallowed sputum, but sensitivity is lower due to intermittent shedding and dilution .

Why Not Other Options?

• c) Urine: Not a diagnostic sample for P. westermani (unlike Schistosoma haematobium).

• d) Skin snips: Used for Onchocerca volvulus microfilariae, not lung fluke eggs