PCAT Microbiology 5 — Questions and Answers
Question 1: Which viral replication strategy characterizes retroviruses and is essential for their integration into the host genome?
- RNA → RNA → Protein
- DNA → RNA → Protein
- RNA → DNA → integrated DNA → RNA (Correct answer)
- ssDNA → dsDNA → RNA
Correct answer: RNA → DNA → integrated DNA → RNA
Retroviruses use reverse transcriptase to convert their RNA genome to dsDNA, which integrase then integrates into the host chromosome as proviral DNA.
Question 2: Coagulase production is used to differentiate Staphylococcus aureus from which of the following?
- Streptococcus pyogenes
- Staphylococcus epidermidis (Correct answer)
- Enterococcus faecalis
- Micrococcus luteus
Correct answer: Staphylococcus epidermidis
S. aureus is coagulase-positive, while S. epidermidis and other coagulase-negative staphylococci lack this enzyme, making coagulase the key differentiating test.
Question 3: The quorum sensing mechanism in bacteria regulates gene expression based on:
- Temperature changes in the environment
- Population density detected via signaling molecules (Correct answer)
- Nutrient availability in the growth medium
- Oxygen levels in the surrounding tissue
Correct answer: Population density detected via signaling molecules
Quorum sensing uses secreted autoinducer molecules that accumulate with cell density; when a threshold is reached, they bind receptors and alter gene expression.
Question 4: Which characteristic of Mycobacterium tuberculosis allows it to survive within macrophage phagosomes?
- Production of catalase to neutralize reactive oxygen species
- Prevention of phagosome-lysosome fusion (Correct answer)
- Secretion of leukocidins that kill macrophages
- Rapid escape from the phagosome into cytoplasm
Correct answer: Prevention of phagosome-lysosome fusion
M. tuberculosis arrests phagosome maturation, preventing acidification and fusion with lysosomes, allowing the bacterium to survive and replicate within macrophages.
Question 5: A patient develops watery diarrhea after antibiotic use; stool test shows Clostridioides difficile toxin. The toxins A and B primarily act by:
- Activating adenylyl cyclase to increase cAMP
- Inactivating Rho GTPases to disrupt the actin cytoskeleton (Correct answer)
- Blocking neuromuscular transmission
- Cleaving host cell ribosomal RNA
Correct answer: Inactivating Rho GTPases to disrupt the actin cytoskeleton
C. difficile toxins A and B glucosylate and inactivate Rho GTPases, disrupting actin dynamics, tight junctions, and epithelial barrier integrity.
Question 6: Which antiviral mechanism of action is used by neuraminidase inhibitors (e.g., oseltamivir) in treating influenza?
- Blocking viral attachment to host cell receptors
- Preventing viral uncoating inside the endosome
- Inhibiting release of new virions from infected cells (Correct answer)
- Suppressing viral RNA polymerase activity
Correct answer: Inhibiting release of new virions from infected cells
Neuraminidase inhibitors prevent influenza neuraminidase from cleaving sialic acid residues, trapping newly assembled virions on the infected cell surface.
Question 7: Serological testing reveals a patient has IgG antibodies to hepatitis B surface antigen (anti-HBs) but no HBsAg and no anti-HBc. This pattern most likely indicates:
- Active hepatitis B infection
- Chronic hepatitis B carrier state
- Successful vaccination against hepatitis B (Correct answer)
- Resolved natural hepatitis B infection
Correct answer: Successful vaccination against hepatitis B
Anti-HBs alone (without anti-HBc) indicates immunity from vaccination, since the vaccine contains only HBsAg and does not induce core antibodies.
PCAT Microbiology 5
Which viral replication strategy characterizes retroviruses and is essential for their integration into the host genome?