MicroBiology MCQs with Answer and Explnation | Chapter 57

Answer: Streptococcus viridans

Streptococcus viridans is a common cause of subacute bacterial endocarditis, especially in individuals with prosthetic heart valves. It is optochin-resistant, catalase-negative, and exhibits alpha-hemolysis.

The other options are incorrect:

• Pseudomonas aeruginosa: This is a gram-negative bacillus, not a gram-positive coccus like Streptococcus viridans. It’s also uncommon in IE.
• Streptococcus pneumoniae: While a gram-positive coccus, S. pneumoniae typically causes pneumonia, not IE.
• Serratia marcescens: Another gram-negative bacillus, rarely associated with IE.

Answer: Oropharynx

Viridans group streptococci are normal flora in the mouth and throat (oropharynx). Subacute bacterial endocarditis (SBE) often occurs when these bacteria enter the bloodstream through routine activities like dental procedures and can then lodge on damaged heart valves, causing infection.

The other options are incorrect:

• Colon: While some strains of streptococci can be found in the gut, viridans group streptococci are not typically associated with the colon and wouldn’t be a common source for SBE.
• Skin: While skin infections can cause bacteremia, viridans group streptococci are not the most common culprit for skin infections that lead to SBE.
• Urethra: The urethra is part of the urinary tract, and viridans group streptococci are not typically found there in healthy individuals. Urinary tract infections are usually caused by other types of bacteria.

Answer: Staphylococcus epidermidis

Staphylococcus epidermidis is a common cause of prosthetic valve endocarditis, especially in patients with a history of cardiac valve replacement. It is catalase-positive and coagulase-negative, consistent with the characteristics described in the scenario.

The other options are incorrect:

• Enterococcus faecalis: While Enterococcus can cause SBE, it is less common than Staphylococcus epidermidis in prosthetic valve infections. It can also be catalase-positive or negative.
• Kingella kingae: This bacteria is more commonly associated with osteoarthiritis and rarely causes SBE.
• Staphylococcus aureus: Although Staphylococcus aureus can cause SBE, it is typically associated with more aggressive symptoms and is less common in prosthetic valve infections compared to Staphylococcus epidermidis. It is also coagulase-positive.

Answer: Mice are the main reservoir of the causative organism.

Lyme disease is primarily transmitted to humans through the bite of infected black-legged ticks (Ixodes scapularis in the northeastern and upper Midwestern United States, and Ixodes pacificus in the western United States). These ticks acquire the causative organism, Borrelia burgdorferi, by feeding on small mammals, particularly mice, which serve as the main reservoir for the bacterium.

The other options are incorrect:

• The Lyme disease vaccine contains toxoid as the immunogen: The Lyme disease vaccine, when it was available, did not contain toxoid. It primarily used recombinant proteins from the outer surface of Borrelia burgdorferi.
• Fleas are the principal mode of transmission of the causative organism: Fleas are not involved in the transmission of Lyme disease. Ticks, specifically Ixodes species, are the primary vectors.
• The causative organism is a small gram-positive rod: Borrelia burgdorferi, the bacterium responsible for Lyme disease, is a spirochete, which is a type of gram-negative bacteria with a spiral shape, not a gram-positive rod.

Answer: Yersinia pestis—flea bite

Yersinia pestis, the bacterium that causes bubonic plague, is primarily transmitted through the bites of infected fleas. Fleas become infected by feeding on rodents carrying the bacteria.

The other options are incorrect:

• Coxiella burnetii—bat guano: While Coxiella burnetii can be present in bat guano, the primary route of transmission to humans is through inhalation of aerosolized bacteria from contaminated environments (e.g., farms, birthing facilities).
• Borrelia burgdorferi—mosquito bite: Borrelia burgdorferi, the causative agent of Lyme disease, is transmitted through the bite of infected Ixodes ticks, not mosquitos.
• Rickettsia rickettsii—contaminated food: Rickettsia rickettsii, the bacteria responsible for Rocky Mountain spotted fever, is transmitted through the bite of infected ticks (usually Dermacentor species). Contaminated food is not a typical transmission route.

Answer: The species of Leptospira that cause leptospirosis grow primarily in humans and are usually transmitted by human-to-human contact.

Leptospira bacteria are carried by animals like rodents and livestock. Humans become infected through contact with contaminated water or soil containing their urine. Transmission directly from human to human is very rare.

The other options are incorrect:

• Species of Borrelia cause a tick-borne disease called relapsing fever: This statement is correct. Borrelia burgdorferi, for example, causes Lyme disease transmitted by ticks.
• Species of Treponema are part of the normal flora of the mouth: This is also correct. Treponema pallidum, for instance, is found in the mouth and causes syphilis.
• Species of Treponema cause syphilis and yaws: Treponema pallidum is indeed the cause of both syphilis and yaws.

Answer: Nose

Staphylococcus aureus is a common bacterium found in the normal flora of the nose and skin, including in healthy newborns. In a newborn nursery outbreak, healthcare workers’ hands contaminated with S. aureus from their own nasal passages are the most likely source of transmission to susceptible newborns.

The other options are incorrect:

• Throat: While S. aureus can be present in the throat, it’s less common than the nose as a carrier site.
• Vagina: S. aureus is not typically part of the normal vaginal flora in healthy women.
• Colon: While some strains of S. aureus can be found in the gut, it’s not the most common location for this specific strain causing a nursery outbreak.

Answer: Brucellosis

Brucellosis is a zoonotic bacterial disease transmitted through direct contact with infected animals or their products (e.g., unpasteurized milk, contaminated meat). It does not involve an arthropod vector like ticks, mosquitoes, or fleas.

The other options are incorrect:

• Lyme disease: Lyme disease is transmitted through the bite of infected Ixodes ticks.
• Plague: Plague can be transmitted in several ways, including through the bite of infected fleas (Yersinia pestis) on rodents.
• Epidemic typhus: This disease is spread through the feces of infected lice (Rickettsia prowazekii).

Answer: Lipid A

Lipid A is a component of the lipopolysaccharide (LPS) on the outer membrane of gram-negative bacteria. LPS is a potent endotoxin that triggers the body’s inflammatory response, leading to fever, chills, and hypotension, which are all symptoms this woman is experiencing.

The other options are incorrect:

• Coagulase: Coagulase is an enzyme produced by some Staphylococcus species and is not directly related to fever, chills, or hypotension.
• Mycolic acid: Mycolic acid is a component of the cell wall of some bacteria, including Mycobacteria (e.g., tuberculosis). While it can contribute to the immune response, it’s not the primary cause of the symptoms described.
• Polysaccharide capsule: A polysaccharide capsule can help some bacteria evade the immune system, but it doesn’t directly cause fever, chills, or hypotension.

Answer: lipopolysaccharide

Lipopolysaccharide (LPS), also known as endotoxin, is a component of the outer membrane of Gram-negative bacteria, including Escherichia coli. It is a potent stimulator of the immune system and can lead to the systemic inflammatory response seen in sepsis and septic shock.

The other options are incorrect:

• Capsule: A polysaccharide capsule can help some bacteria evade phagocytosis by immune cells, but it doesn’t directly cause the severe inflammatory response seen in septic shock.
• Teichoic acid: Teichoic acid is found in the cell wall of gram-positive bacteria, not gram-negative E. coli.
• Pili: Pili are hair-like structures on bacteria that help them adhere to host cells. While they can play a role in infection, they are not directly responsible for septic shock.

Answer: Mouth

Viridans group streptococci are normal flora found in the mouth and throat (oropharynx). This makes the mouth the most likely source of the organism that caused endocarditis in this patient.

The other options are incorrect:

• Colon: While some strains of streptococci can be found in the gut, viridans group streptococci are not typically associated with the colon and wouldn’t be a common source for endocarditis.
• Skin: While skin infections can cause bacteremia (bacteria in the bloodstream), viridans group streptococci are not the most common culprit for skin infections that lead to endocarditis.
• Stomach: The stomach environment is harsh due to strong stomach acid. Viridans group streptococci are not typically found in the stomach and wouldn’t survive there for long.

Answer: Arthropod vector

The patient’s symptoms are highly suggestive of Lyme disease, which is caused by the bacterium Borrelia burgdorferi. This bacterium is transmitted through the bite of infected ticks, which are classified as arthropods.

The other options are incorrect:

• Respiratory route: Lyme disease is not spread through the respiratory route (coughing, sneezing).
• Sexual contact: Lyme disease is not a sexually transmitted infection.
• Consumption of contaminated food: Consuming contaminated food or water is not a transmission route for Lyme disease.

Answer: Brucellae infect reticuloendothelial cells in the liver, spleen, and bone marrow.

Brucellae, the bacteria that cause brucellosis, have a tropism for reticuloendothelial cells, which are immune cells found in various organs, including the liver, spleen, and bone marrow. These cells play a role in the immune response and phagocytosis (engulfing foreign particles). Brucellae can survive and replicate within these cells.

The other options are incorrect:

• Brucellae are transmitted primarily by tick bite: Brucellosis is a zoonotic disease primarily transmitted through contact with infected animals or their products (unpasteurized milk, contaminated meat). Ticks are not the main culprits.
• The principal reservoirs of Brucellae are small rodents: While rodents can carry Brucella species, the main reservoirs for human infection are livestock like cattle, sheep, and goats.
• Brucellae are obligate intracellular parasites that are usually identified by growth in human cell culture: Brucellae can survive and replicate inside host cells but are not obligate intracellular parasites (meaning they can survive outside cells too). They can be grown in specific culture media suitable for their needs, not necessarily human cell cultures.

Answer: people

Brucellae, the bacteria responsible for brucellosis, have a tropism for reticuloendothelial cells, particularly those found in the liver, spleen, and bone marrow. This characteristic contributes to the systemic manifestations of the disease.

The other options are incorrect:

• Dogs and cats: While Salmonella can infect pets, Salmonella Typhi specifically is not commonly found in these animals and wouldn’t be a likely source for this large outbreak.
• Turkeys: While Salmonella can contaminate poultry products like turkey, Salmonella Typhi is not typically associated with outbreaks from this source.
• Water: Contaminated water can be a source of typhoid fever, but in this case, the residents having meals from the same kitchen suggests a foodborne route is more likely.

Answer: The causative organism forms beta-hemolytic colonies on blood agar.

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a bacterium that belongs to the genus Rickettsia. These bacteria are obligate intracellular parasites, meaning they cannot grow on their own outside a host cell. Therefore, they cannot form colonies on blood agar or any other standard culture media.

The other options are incorrect:

• The disease is caused by a Rickettsia: This statement is correct. Rocky Mountain spotted fever is indeed caused by the bacterium Rickettsia rickettsii.
• Headache, fever, and rash are characteristic features of the disease: This statement is correct. Headache, fever, and rash are hallmark symptoms of Rocky Mountain spotted fever.
• The disease occurs primarily east of the Mississippi: This statement is correct. Rocky Mountain spotted fever is more commonly reported in the eastern United States, particularly in regions east of the Mississippi River.

Answer:

Epidemic typhus is caused by the bacterium Rickettsia prowazekii. This bacterium is transmitted through the feces of infected human body lice, not ticks.pen_spark

The other options are incorrect:

• The disease is caused by a Rickettsia: This statement is correct. Rickettsia prowazekii is indeed a Rickettsia species.
• The Weil-Felix test can aid in diagnosis of the disease: This statement is correct. The Weil-Felix test, although not specific, can be used as a supportive test for diagnosing epidemic typhus by detecting antibodies against related bacteria (Proteus species).
• The disease is characterized by a rash: This statement is also correct. A rash is a characteristic feature of epidemic typhus.

Answer: Enterococcus faecalis

Enterococcus faecalis is most likely responsible for bacterial endocarditis in men following urological instrumentation. This bacterium is commonly found in the gastrointestinal tract and can enter the bloodstream during urological procedures, leading to endocarditis, particularly in patients with preexisting heart valve abnormalities.

The other options are incorrect:

• Ureaplasma urealyticum: While Ureaplasma can be found in the urogenital tract, it is not a typical cause of endocarditis.
• Streptococcus pyogenes (Group A Strep): This bacteria is more commonly associated with skin infections and “strep throat” and is less likely to cause endocarditis following urological procedures.
• Pseudomonas aeruginosa: This bacterium is a common cause of hospital-acquired infections but is less frequent in endocarditis compared to Enterococcus faecalis, especially after urological procedures.

Answer: Epidemic typhus

Ticks are vectors for the transmission of various diseases, including tularemia, Lyme disease, and Rocky Mountain spotted fever. However, epidemic typhus is primarily transmitted by body lice, not ticks. Epidemic typhus is caused by the bacterium Rickettsia prowazekii and is associated with crowded and unsanitary conditions rather than tick habitats.

The other options are incorrect:

• Tularemia: Ticks can transmit tularemia through bites. This bacterial infection can cause fever, chills, and muscle aches.
• Lyme disease: This common tick-borne disease is caused by bacteria and can cause a characteristic bull’s-eye rash, fatigue, and joint pain.
• Rocky Mountain spotted fever: This serious tick-borne disease can cause fever, rash, and headache.

Answer: Rash is a prominent feature.

While a rash can occur in some cases of Q fever, it is not a prominent or defining feature of the disease. Q fever is more commonly associated with flu-like symptoms, such as fever, fatigue, muscle aches, and headache.

The other options are incorrect:

• Farm animals are an important reservoir: This statement is correct. Farm animals, particularly goats, sheep, and cattle, are a significant reservoir for Coxiella burnetii, the bacteria that causes Q fever.
• It is transmitted by respiratory aerosol: This statement is correct. Inhalation of contaminated aerosols is the primary mode of transmission for Q fever.
• It is caused by Coxiella burnetii: This statement is correct. Coxiella burnetii is the bacteria responsible for causing Q fever.

Answer: Sleeping sickness can be diagnosed by finding eggs in the stool.

Sleeping sickness, also known as African trypanosomiasis, is caused by the parasite Trypanosoma brucei, which is transmitted by the bite of infected tsetse flies. However, the presence of eggs in the stool is not a diagnostic feature of sleeping sickness. Instead, diagnosis typically involves detecting the parasite in blood, lymph fluid, or cerebrospinal fluid samples.

The other options are incorrect:

• Sleeping sickness is caused by a trypanosome: This statement is correct. Trypanosoma brucei, a parasite, is the culprit behind sleeping sickness.
• Sleeping sickness occurs primarily in tropical Africa: This statement is correct. The tsetse fly, which transmits the parasite, is found only in sub-Saharan Africa.
• Sleeping sickness is transmitted by tsetse flies: This statement is correct. The bite of an infected tsetse fly is the primary mode of transmission for sleeping sickness.

Answer: Rickettsia rickettsii

Rickettsia rickettsii is a type of bacteria belonging to the order Rickettsiales, known for primarily infecting vascular endothelial cells. These cells line the inside of blood vessels and play a crucial role in blood flow and regulation. Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever.

The other options are incorrect:

• Haemophilus influenzae: This bacterium primarily infects the respiratory tract, causing illnesses like pneumonia and bronchitis. It doesn’t target vascular endothelial cells.
• Salmonella typhi: This bacterium is responsible for typhoid fever and mainly infects the gastrointestinal tract. It doesn’t have a particular affinity for vascular endothelial cells.
• Coxiella burnetii: This bacterium causes Q fever and can infect various cell types, including the lungs. However, it’s not known to primarily target vascular endothelial cells.

Answer: Penicillin is a bacteriostatic drug because autolytic enzymes are not activated.

Penicillin is actually considered a bactericidal drug in most cases. It works by inhibiting transpeptidases, enzymes needed for bacterial cell wall synthesis. This weakened cell wall makes the bacteria susceptible to bursting from its internal pressure.

The other options are incorrect:

• Penicillin inhibits transpeptidases, which are required for cross-linking peptidoglycan: This statement is correct. This is the primary mechanism of action for penicillin.
• The structure of penicillin resembles that of a dipeptide of alanine, which is a component of peptidoglycan: This statement is correct. This structural similarity allows penicillin to bind to the transpeptidases and block their function.
• An intact ß-lactam ring of penicillin is required for its activity: This statement is correct. The ß-lactam ring is essential for penicillin’s ability to bind to and inhibit transpeptidases.

Answer: protein synthesis.

Ribosomes are the cellular machinery responsible for protein synthesis. When antibiotics inhibit or inactivate ribosomes, they directly disrupt this essential process. Without protein synthesis, bacteria cannot grow, repair, or reproduce, ultimately leading to cell death.

The other options are incorrect:

• ATP production: Ribosomes are not involved in ATP production, which primarily occurs in the mitochondria. The mitochondria use cellular respiration to generate ATP, the cell’s main energy source.
• Cell division: Protein synthesis is necessary for cell division, as new proteins are required for various cellular components involved in the division process. However, ribosome inhibition specifically targets protein synthesis, not cell division itself.
• DNA replication: Ribosomes are not directly involved in DNA replication. This process relies on DNA polymerases and other enzymes to copy the genetic material.

Answer: Aminoglycosides are bacteriostatic drugs that inhibit protein synthesis by activating ribonuclease, which degrades mRNA.

This statement is incorrect. Aminoglycosides are actually bactericidal drugs, not bacteriostatic. They work by binding to the 30S ribosomal subunit of bacterial ribosomes, leading to misreading of the mRNA code and ultimately causing the synthesis of faulty proteins, which results in bacterial cell death.

The other options are incorrect:

• Cephalosporins are bactericidal drugs that inhibit the transpeptidase reaction and prevent cell wall synthesis: This statement is correct. Similar to penicillin, cephalosporins work by inhibiting enzymes needed for bacterial cell wall formation.
• Erythromycin is a bacteriostatic drug that inhibits protein synthesis by blocking translocation of the polypeptide: This statement is correct. Erythromycin binds to the ribosome and disrupts the process of transferring amino acids to the growing protein chain.
• Tetracyclines are bacteriostatic drugs that inhibit protein synthesis by blocking tRNA binding: This statement is correct. Tetracyclines prevent the attachment of transfer RNA (tRNA) to the ribosome, which is crucial for protein synthesis.

Answer: Resistance genes are rarely transferred by conjugation.

Conjugation is a well-documented mechanism for horizontal gene transfer among bacteria, including the spread of resistance genes. Plasmids (R factors) and transposons containing resistance genes can be readily transferred between bacteria through conjugation.

The other options are incorrect:

• Resistance to some drugs is due to a chromosomal mutation that alters the receptor for the drug: This statement is correct. Mutations in chromosomal genes can modify drug receptors, reducing the drug’s binding and effectiveness.
• R factors are plasmids that carry the genes for enzymes that modify one or more drugs: This statement is correct. R factors (resistance factors) are plasmids that can harbor genes encoding enzymes that inactivate or modify antibiotics, making them ineffective.
• Resistance to some drugs is due to transposon genes that code for enzymes that inactivate the drugs: This statement is correct. Transposons are mobile genetic elements that can carry resistance genes and integrate into chromosomal or plasmid DNA. They can encode enzymes that modify or destroy antibiotics.

Answer: They consist of RNA without a protein or lipoprotein outer coat.

This statement is the most accurate description of viroids. Viroids are small infectious agents composed solely of short, circular, single-stranded RNA molecules without any protein or lipoprotein outer coat. They are simpler in structure compared to viruses, lacking the protein coat found in viruses.

The other options are incorrect:

• They cause tumors in experimental animals: While some viroids can cause plant diseases with a stunting or abnormal growth appearance, they are not directly linked to tumor formation in animals.
• They are defective viruses that are missing the DNA coding for the matrix protein: Viroids are distinct from viruses. Viruses typically have a protein coat and often contain DNA or RNA as their genetic material. Viroids are much simpler and solely consist of RNA.
• They require an RNA polymerase in the particle for replication to occur: Viroids don’t carry their own enzymes for replication. They rely on the host plant’s cellular machinery, including RNA polymerase, to replicate their RNA molecules.

Answer: Virus-mediated inhibition of host DNA synthesis

While some viruses can suppress host cell functions, including DNA synthesis, this doesn’t directly contribute to viral persistence within an infected individual. In some cases, it might even limit viral replication by hindering host cell division needed for virus production.

The other options are incorrect:

• Generation of defective-interfering particles: These are incomplete viral particles that can interfere with the replication of complete viruses within the host, potentially limiting the spread of the virus but not necessarily eliminating it entirely.
• Host tolerance to viral antigens: Over time, the immune system might develop a level of tolerance to certain viral antigens, allowing the virus to persist in a low-level, chronic infection.
• Integration of a provirus into the genome of the host: This is a strategy used by retroviruses like HIV. Integration allows the viral genome to become part of the host’s DNA, enabling the virus to persist as long as the infected cell survives and replicates.

Answer: A heterophil antibody is formed that agglutinates sheep or horse red blood cells.

The heterophil antibody test is a common method for diagnosing infectious mononucleosis (caused by Epstein-Barr Virus, EBV). This test detects the presence of heterophil antibodies, which are antibodies produced by the immune system that can non-specifically agglutinate (clump together) red blood cells from sheep or horses. While they don’t directly target EBV, their presence indicates an immune response consistent with EBV infection.

The other options are incorrect:

• A heterophil antigen is formed that reacts with a capsid protein of Epstein–Barr virus: Heterophils are antibodies, not antigens. Antigens are molecules the immune system recognizes and reacts to. EBV does have capsid proteins, but the test doesn’t detect a specific reaction with them.
• A heterophil antigen occurs following infection with cytomegalovirus: While cytomegalovirus can cause similar symptoms to infectious mononucleosis, the heterophil antibody test is primarily associated with EBV infection.
• A heterophil antigen occurs that cross-reacts with Proteus OX19 strains: Proteus OX19 is a bacterial strain. While the heterophil antibody test can sometimes cross-react with some bacterial antigens, it’s not specific to Proteus OX19.

Answer: Respiratory droplet spread

Measles is a highly contagious virus that spreads primarily through respiratory droplets. When an infected person coughs, sneezes, or talks, infected droplets are expelled into the air. These droplets can be inhaled by others who are close by, leading to infection.

The other options are incorrect:

• Animal bite: Measles is a virus specific to humans and doesn’t spread through animal bites.
• Fomite spread: While the measles virus can survive on surfaces for a short time (up to two hours), respiratory droplet spread is the primary mode of transmission. Fomite spread is less likely compared to airborne transmission.
• Tick bite: Measles is not transmitted by ticks. Ticks are vectors for other diseases like Lyme disease and Rocky Mountain spotted fever.

Answer: Respiratory syncytial virus

The presence of giant multinucleated cells in the sputum analysis is a strong indicator of RSV infection. This is a common respiratory virus that can cause wheezing, difficulty breathing, cough, and rhinorrhea (runny nose) in young children.

The other options are incorrect:

• Measles: While measles can cause respiratory symptoms, it wouldn’t typically present with giant multinucleated cells in the sputum. Measles is also less common at 11 months of age as most children receive the MMR vaccine around 12 months.
• B19 (Parvovirus B19): Parvovirus B19, also known as fifth disease, typically presents with a characteristic “slapped cheek” rash and doesn’t cause the specific respiratory symptoms or giant multinucleated cells seen in RSV infection.
• Parainfluenza: Similar to RSV, parainfluenza can cause respiratory symptoms in children. However, the presence of giant multinucleated cells is less common with parainfluenza compared to RSV.

Answer: The viral genome consists of three segments of double-stranded RNA.

This statement is incorrect. Retroviruses do not have a genome consisting of three segments of double-stranded RNA. Instead, retroviruses have a single-stranded RNA genome. The other three statements accurately describe various aspects of retrovirus biology.

The other options are incorrect:

• During infection, the virus synthesizes a DNA copy of its RNA, and this DNA becomes covalently integrated into host cell DNA: This is a defining characteristic of retroviruses. An enzyme called reverse transcriptase uses the viral RNA as a template to create a complementary DNA copy, which then integrates into the host genome.
• The virion is enveloped and enters cells via an interaction with specific receptors on the host cell: This statement is correct. Retroviruses have an envelope with specific proteins that can bind to receptors on host cells, facilitating entry.
• The virion carries an RNA-directed DNA polymerase encoded by the viral genome: This statement is also correct. The enzyme reverse transcriptase, crucial for creating the DNA copy from viral RNA, is encoded by the viral genome and carried within the virion.

Answer: The interaction determines whether the purified genome of a virus is infectious.

Viral adsorption is the initial attachment of the virus to the host cell surface. While it plays a role in determining which cells a virus can infect, it doesn’t directly determine the infectivity of the purified viral genome itself.

The other options are incorrect:

• If the sites are occupied, interference with virus infection occurs: This is true. If the specific receptor sites on the cell surface are already occupied by other molecules or viruses, it can hinder further viral attachment and infection.
• The interaction can be prevented by neutralizing antibody: This is true. Neutralizing antibodies can bind to the viral envelope proteins, blocking their interaction with the cell receptor and preventing attachment.
• The interaction determines the specific target organs for infection: This is true. The specific receptors present on different cell types determine which organs or tissues a virus can target and infect.

Answer: Enveloped viruses use a matrix protein to mediate interactions between viral glycoproteins in the plasma membrane and structural proteins in the nucleocapsid.

This statement is correct. Enveloped viruses possess a matrix protein located beneath their lipid envelope. The matrix protein helps to stabilize the viral structure and mediates interactions between the viral glycoproteins embedded in the envelope and the structural proteins present in the nucleocapsid, aiding in virion assembly and budding.

The other options are incorrect:

• The symmetry of virus particles prevents inclusion of any nonstructural proteins, such as enzymes: Viral symmetry refers to the arrangement of capsid subunits. While it dictates the overall structure, it doesn’t necessarily exclude nonstructural proteins. Some viruses can package enzymes needed for replication within the capsid alongside the genome.
• Helical nucleocapsids are found primarily in DNA viruses: Helical nucleocapsids, where the nucleic acid forms a helical coil surrounded by capsid proteins, can occur in both RNA and DNA viruses. The type of nucleic acid doesn’t dictate the nucleocapsid shape in all cases.
• Most viruses acquire surface glycoproteins by budding through the nuclear membrane: While some viruses can bud through the nuclear membrane, the more common site for enveloped virus budding is the endoplasmic reticulum or Golgi apparatus of the host cell. These cellular compartments are involved in protein processing and glycosylation (adding sugar molecules), which are important for viral glycoproteins.

Answer: Viruses replicate by binary fission.

Viruses do not reproduce by binary fission, which is a type of asexual reproduction seen in bacteria. Viruses have a unique replication cycle that involves hijacking the host cell’s machinery to create new viral particles.

The other options are incorrect:

• The proteins on the surface of the virus mediate the entry of the virus into host cells: This statement is correct. Viral surface proteins, often called glycoproteins, play a key role in attaching to specific receptors on host cells, facilitating entry.
• Neutralizing antibody is directed against proteins on the surface of the virus: This statement is correct. The immune system can produce antibodies that specifically target viral surface proteins, preventing them from binding to host cells and rendering them non-infectious.
• Viruses can reproduce only within cells: This statement is correct. Viruses are obligate intracellular parasites, meaning they require a host cell to replicate. They lack the cellular machinery needed for independent reproduction

Answer: Cytomegalovirus

Cytomegalovirus (CMV) is one of the most common viruses that can be transmitted from mother to baby during pregnancy (congenitally). It can be passed through the placenta or during childbirth through vaginal secretions. While other viruses on the list can also be transmitted in utero, CMV is the most prevalent in the U.S.

The other options are incorrect:

• Respiratory syncytial virus (RSV): RSV is a common respiratory virus typically acquired through respiratory droplets after birth, not in utero.
• Rubella virus: Rubella can be transmitted during pregnancy, but due to widespread vaccination programs, its incidence is much lower compared to CMV in the U.S.
• Hepatitis B virus: Hepatitis B can be transmitted from mother to baby during childbirth, but with proper vaccination of mothers, the risk is significantly reduced.

Answer: Their virions contain an RNA polymerase.

This statement is incorrect. Neither the measles virus nor the rubella virus contains an RNA polymerase within their virions. RNA polymerase is a viral enzyme necessary for replication of the viral RNA genome inside host cells, but it is not typically packaged within the virion structure of these viruses.

The other options are incorrect:

• They are transmitted by respiratory aerosol: Both the measles virus and the rubella virus are transmitted by respiratory aerosols, primarily through coughing and sneezing of infected individuals. This mode of transmission contributes to the high infectivity of these viruses.
• They have a single antigenic type: Both the measles virus and the rubella virus have multiple antigenic types. For instance, there are different strains of measles virus circulating globally, and immunity to one strain may not confer immunity to other strains. Similarly, rubella virus strains can exhibit antigenic variation.
• They are RNA enveloped viruses: Both the measles virus and the rubella virus are RNA viruses that possess an envelope derived from the host cell membrane during the process of budding from infected cells. This envelope contains viral glycoproteins involved in viral entry and immune evasion.

Answer: Chickenpox due to the varicella-zoster virus

The symptoms described, including fever and a generalized macular rash evolving rapidly into vesicles and then pustules, are characteristic of chickenpox, caused by the varicella-zoster virus (VZV). Chickenpox typically presents with multiple waves of lesions appearing over several days, primarily affecting the scalp and trunk before spreading to other parts of the body.

The other options are incorrect:

• Whitlow infection due to herpes simplex virus type 1: Whitlow is a localized infection of the finger or thumb caused by herpes simplex virus (HSV) type 1 or type 2. It typically presents as painful vesicles or pustules on the affected digit, often in individuals with a history of oral or genital herpes infection. The described symptoms are not consistent with whitlow.
• Infectious mononucleosis due to the Epstein-Barr virus: Infectious mononucleosis, caused by the Epstein-Barr virus (EBV), typically presents with symptoms such as fever, sore throat, swollen lymph nodes, and fatigue. While EBV infection can cause a rash, it usually differs from the rapid evolution of lesions seen in the described case.
• Exanthem subitum due to cytomegalovirus: Exanthem subitum, also known as roseola infantum, is a viral illness caused by human herpesvirus 6 (HHV-6) or, less commonly, human herpesvirus 7 (HHV-7). It is characterized by a high fever followed by the sudden onset of a rash once the fever resolves. The rash typically consists of pink, flat or slightly raised lesions and is not associated with the rapid evolution into vesicles and pustules seen in chickenpox.

Answer: It results in new sub types over time

Antigenic drift in influenza viruses refers to the gradual accumulation of mutations in the genes encoding surface proteins (hemagglutinin – HA and neuraminidase – NA). These mutations cause slight changes in the protein structure, allowing the virus to partially evade the immune system’s recognition from previous exposure or vaccination. Over time, this antigenic drift can lead to the emergence of new subtypes that are antigenically distinct from previous circulating strains. This necessitates regular updates to influenza vaccines to maintain effectiveness.

The other options are incorrect:

• It results in major antigenic changes: While drift can lead to reduced vaccine effectiveness, the changes are typically gradual, not major.
• It is exhibited by only influenza A viruses: Antigenic drift can occur in both influenza A and influenza B viruses.
• It is due to frame shift mutations in viral genes: Frame shift mutations are a specific type of mutation that can significantly alter protein structure. While they can occur, antigenic drift is more commonly caused by point mutations (single nucleotide changes) in the viral genes.

Answer: Poliovirus

Poliovirus is a single-stranded positive-sense RNA virus. Positive-sense RNA viruses have a unique characteristic: their RNA genome itself is infectious. This means the viral RNA can directly enter a host cell and be translated by the cellular machinery to produce viral proteins, initiating infection.

The other options are incorrect:

• Influenza virus: Influenza virus also has a single-stranded RNA genome, but it’s negative-sense RNA. Negative-sense RNA needs to be converted into positive-sense RNA first within the host cell before it can be used for protein synthesis.
• Measles virus: Measles virus has a single-stranded negative-sense RNA genome, similar to influenza.
• Rotavirus: Rotavirus has a segmented double-stranded RNA genome. Double-stranded RNA needs specific enzymes for replication within the host cell and wouldn’t be directly infectious on its own.

Answer: Acyclovir reduces the number of recurrent disease episodes by eradicating latently infected cells.

Acyclovir is an antiviral medication that can’t eliminate the herpes virus completely. It works by stopping the virus from multiplying, but it doesn’t eradicate the infected cells where the virus lies dormant.

The other options are incorrect:

• Initial disease episodes are generally more severe than recurrent episodes. – This statement is mostly accurate. Initial outbreaks tend to be more severe with longer healing times.
• Multinucleated giant cells with intranuclear inclusions are found in the lesions. – This is also accurate. These are characteristic signs of herpes infection in tissue samples.
• Genital herpes can be transmitted in the absence of apparent lesions. – This statement is accurate as well. The virus can be shed from the skin even when there are no visible sores.

Answer: The protein involved in antigenic drift is primarily the internal ribonucleoprotein.

Antigenic drift is a gradual change in the surface proteins (hemagglutinin – HA and neuraminidase – NA) of the influenza A virus. The internal ribonucleoprotein is not involved in this process.

The other options are incorrect:

• Antigenic shifts, which represent major changes in antigenicity, occur infrequently and are due to the reassortment of segments of the viral genome. – This statement is correct. Reassortment happens when different influenza A strains infect a single host cell, allowing their RNA segments to mix and create entirely new viruses with significantly altered antigenicity.
• Antigenic shifts affect both the hemagglutinin and the neuraminidase. – This statement is also correct. Since these are surface proteins, mutations in them significantly impact how the immune system recognizes the virus.
• The worldwide epidemics caused by influenza A virus are due to antigenic shifts. – This statement is generally accurate. Major pandemics are often caused by antigenic shifts, as they create viruses with entirely new surface proteins that a large portion of the population has no immunity against.

Answer: Some new viral antigens appear on the surface of the host cell.

While viruses can utilize host cell machinery for protein production, they don’t directly display new viral antigens on the host cell surface. Viruses have their own envelope or capsid containing viral proteins that the immune system recognizes as foreign.

The other options are incorrect:

• Some of these viruses contain host cell lipids. – This statement is accurate. Viral envelopes, found in some viruses, incorporate host cell lipids along with viral proteins.
• Some of these viruses cause multinucleated giant cell formation. – This is also correct. Some viruses, like herpesviruses, can cause infected cells to fuse, resulting in giant cells with multiple nuclei.
• Some of these viruses do not have an envelope. – This statement is accurate as well. Viruses can be classified into enveloped (with a lipid bilayer) or non-enveloped (naked capsid).

Answer: Coxsackie virus

Coxsackieviruses are single-stranded RNA viruses. They belong to the Picornaviridae family, known for their positive-sense RNA genomes.

The other options are incorrect:

• Adenovirus: Adenoviruses have a linear, double-stranded DNA genome.
• Rotavirus: Rotaviruses possess a segmented double-stranded RNA genome.
• Herpes simplex virus: Herpes simplex viruses (HSV-1 & HSV-2) have a double-stranded DNA genome.

Answer: Viruses must degrade host cell DNA in order to obtain nucleotides.

Viruses can utilize nucleotides from the host cell cytoplasm for their replication, without necessarily degrading the host DNA. They can acquire these nucleotides from breakdown of host RNA or other cellular processes.

The other options are incorrect:

• Viruses cannot synthesize proteins outside of cells. – This is accurate. Viruses lack the ribosomes and other machinery needed for protein synthesis and rely on the host cell’s machinery.
• Viruses cannot generate energy outside of cells. – This statement is correct as well. Viruses lack the metabolic machinery to generate ATP (energy) on their own and rely on host cell processes.
• Enveloped viruses require host cell membranes to obtain their envelopes. – This statement is accurate. Enveloped viruses hijack host cell membranes during their replication process to form their viral envelopes.

Answer: Most infections are symptomatic and require treatment with amphotericin B.

While Coccidioidomycosis (caused by C. immitis) can cause symptoms, a large portion (around 60%) of infections are asymptomatic. Additionally, fluconazole or itraconazole are the preferred antifungals for most cases, with amphotericin B reserved for severe or unresponsive infections.

The other options are incorrect:

• In the body, spherules containing endospores are formed. – This statement is correct. C. immitis transforms into spherules with endospores within the host, which are the infectious propagules.
• A rising titer of complement-fixing antibody indicates disseminated disease. – This statement is also correct. Rising titers of specific antibodies can be indicative of a more widespread infection.
• The mycelial phase of the organism grows primarily in the soil, which is its natural habitat. – This statement is accurate as well. C. immitis exists as a mold (mycelial phase) in the soil.

Answer: Passive immunity in the form of high titer antibodies should be given to those known to be exposed.

Passive immunity through antibodies is not typically used for fungal infections like histoplasmosis. It’s more effective for preventing bacterial infections. Antifungal medications are the primary treatment for histoplasmosis.

The other options are incorrect:

• The natural habitat of H. capsulatum is the soil, where it grows as a mold. – This statement is correct. H. capsulatum lives in the environment, particularly in soil enriched with bird or bat droppings.
• Within the body, H. capsulatum grows primarily intracellularly within macrophages. – This statement is also correct. Upon inhalation, H. capsulatum transforms into a yeast form and resides within macrophages.
• H. capsulatum is transmitted by airborne conidia, and its initial site of infection is the lung. – This statement is accurate as well. People inhale the fungal spores (conidia) which lodge in the lungs, initiating the infection.

Answer: Candida albicans

The presentation described, including red and painful skin folds with creamy white material at the base and microscopic examination revealing oval budding structures, is consistent with a candidal infection. Candida albicans, a yeast fungus, commonly causes intertrigo in skin folds, particularly in obese and diabetic individuals.

The other options are incorrect:

• Epidermophyton floccosum: While this fungus can cause tinea infections (dermatophytosis), it’s less commonly associated with intertrigo.
• Microsporum canis: This fungus primarily causes tinea infections of the hair and beard and wouldn’t typically present with the described symptoms.
• Aspergillus fumigatus: This is a mold causing invasive pulmonary aspergillosis and wouldn’t be found in the context of a skin infection like intertrigo.

Answer: It is an encapsulated non dimorphic yeast found worldwide

The description matches Cryptococcus neoformans, an encapsulated yeast commonly found worldwide. It can cause meningitis in immunocompromised individuals, such as those with HIV/AIDS. Cryptococcus neoformans is visualized by India ink staining in the cerebrospinal fluid (CSF) obtained from lumbar puncture.

The other options are incorrect:

• It exists as a mycelial form at room temperature and a yeast at 37°C: This statement describes the dimorphic nature of fungi like Histoplasma capsulatum or Blastomyces dermatitidis, but not Cryptococcus neoformans, which remains yeast-like at both room temperature and body temperature.
• It can also be seen as “spaghetti and meatballs” on KOH stain: “Spaghetti and meatballs” appearance on KOH stain is characteristic of Malassezia furfur, a lipophilic yeast, and is not seen with Cryptococcus neoformans.
• It is a non-encapsulated dimorphic yeast that reproduces by budding: Cryptococcus neoformans is encapsulated and does not undergo dimorphism. It remains yeast-like in morphology and reproduces by budding

Answer: Both fungi and protozoa use flagella as their organ of motility.

While some protozoa may use flagella for motility, fungi do not possess flagella for movement. Instead, they utilize various structures such as hyphae or spores for dispersal.

The other options are incorrect:

• Both fungi and protozoa generate energy in mitochondria: This statement is correct. Both fungi and protozoa are eukaryotic organisms that generate energy through oxidative phosphorylation in mitochondria.
• Fungi possess a cell wall, whereas protozoa do not: This statement is generally correct. Fungi typically have a rigid cell wall composed of chitin or cellulose, while protozoa lack a cell wall and instead have a plasma membrane.
• Both fungi and protozoa are eukaryotic organisms: This statement is correct. Both fungi and protozoa are eukaryotic organisms, meaning their cells contain membrane-bound organelles and a true nucleus.

Answer: Candidiasis, cryptococcosis, and sporotrichosis

Fungal cells that reproduce by budding are characteristic of Candida species (candidiasis), Cryptococcus neoformans (cryptococcosis), and Sporothrix schenckii (sporotrichosis). These fungi are known to produce budding yeast cells in infected tissues.

The other options are incorrect:

• Sporotrichosis, mycetoma, and aspergillosis: As mentioned above, mycetoma can have fungal causes that don’t involve budding, and aspergillosis is caused by Aspergillus fumigatus, a mold that reproduces by spores, not budding.
• Tinea corporis, tinea unguium, and tinea versicolor: These are all types of dermatophytosis (fungal infections of the skin, hair, and nails). They are caused by dermatophytes, which are molds and reproduce by spores, not budding.
• Candidiasis, cryptococcosis, and aspergillosis: While candidiasis is correct, cryptococcosis is caused by Cryptococcus neoformans, a yeast that reproduces by budding, but aspergillosis, as mentioned before, is caused by a mold reproducing by spores.