NBME Microbiology 3 — Questions and Answers
Question 1: A 4-year-old presents with a barking cough and inspiratory stridor. The causative virus replicates in respiratory epithelium and has a negative-sense, single-stranded RNA genome with a helical nucleocapsid. Which virus family does this pathogen belong to?
- Paramyxoviridae (Correct answer)
- Orthomyxoviridae
- Picornaviridae
- Coronaviridae
Correct answer: Paramyxoviridae
Parainfluenza virus, the most common cause of croup (laryngotracheobronchitis), belongs to Paramyxoviridae and has a negative-sense ssRNA genome with helical symmetry.
Question 2: A retrovirus integrates its genome into the host cell's DNA. Which enzyme is responsible for converting the viral RNA genome into double-stranded DNA prior to integration?
- Reverse transcriptase (Correct answer)
- Integrase
- Protease
- RNA-dependent RNA polymerase
Correct answer: Reverse transcriptase
Reverse transcriptase (RNA-dependent DNA polymerase) copies the viral ssRNA into cDNA and then synthesizes the complementary strand to form dsDNA.
Question 3: A patient with AIDS presents with ring-enhancing lesions in the brain on MRI. CSF India ink preparation is negative. Brain biopsy shows tissue cysts containing bradyzoites. Which organism is responsible?
- Toxoplasma gondii (Correct answer)
- Cryptococcus neoformans
- Aspergillus fumigatus
- JC virus
Correct answer: Toxoplasma gondii
Toxoplasma gondii reactivation in AIDS causes multiple ring-enhancing lesions with bradyzoite-containing tissue cysts; India ink highlights Cryptococcus, not Toxoplasma.
Question 4: A hepatitis virus has a partially double-stranded circular DNA genome and replicates via an RNA intermediate using a viral reverse transcriptase. Which hepatitis virus has this replication strategy?
- Hepatitis B virus (Correct answer)
- Hepatitis C virus
- Hepatitis A virus
- Hepatitis D virus
Correct answer: Hepatitis B virus
Hepatitis B virus (HBV) is a hepadnavirus with a partially dsDNA genome; it replicates through an RNA pregenome intermediate using reverse transcriptase, making it targetable by nucleoside analogs.
Question 5: A 22-year-old college student develops fever, pharyngitis, and posterior cervical lymphadenopathy. A peripheral blood smear shows atypical lymphocytes. The heterophile antibody test is positive. Which cell type is the primary reservoir for the causative virus?
- B lymphocytes (Correct answer)
- T lymphocytes
- Monocytes
- Epithelial cells
Correct answer: B lymphocytes
Epstein-Barr virus (EBV) infects B lymphocytes via CD21 (complement receptor 2); the atypical lymphocytes seen on smear are reactive cytotoxic T cells responding to infected B cells.
Question 6: Interferons are produced by virally infected cells and provide antiviral protection by which primary mechanism?
- Inducing 2'-5' oligoadenylate synthetase and PKR to degrade viral RNA and inhibit translation (Correct answer)
- Activating complement to lyse infected cells directly
- Stimulating antibody production against viral capsid proteins
- Blocking viral fusion proteins from inserting into the host membrane
Correct answer: Inducing 2'-5' oligoadenylate synthetase and PKR to degrade viral RNA and inhibit translation
Type I interferons (IFN-α/β) induce enzymes such as 2'-5' oligoadenylate synthetase (activating RNase L) and PKR, which degrade viral RNA and halt protein synthesis in infected and neighboring cells.
Question 7: A neonate develops encephalitis and diffuse vesicular skin lesions. The mother had no active lesions at delivery. The causative virus establishes latency in the dorsal root ganglia and is a double-stranded DNA virus with an icosahedral capsid surrounded by a tegument and envelope. Which virus is responsible?
- Herpes simplex virus type 2 (Correct answer)
- Varicella-zoster virus
- Cytomegalovirus
- Herpes simplex virus type 1
Correct answer: Herpes simplex virus type 2
Neonatal herpes is most often caused by HSV-2 acquired during passage through an infected birth canal, even without visible maternal lesions; it causes encephalitis and disseminated vesicular lesions.
NBME Microbiology 3
A 4-year-old presents with a barking cough and inspiratory stridor.
The causative virus replicates in respiratory epithelium and has a negative-sense, single-stranded RNA genome with a helical nucleocapsid.
Which virus family does this pathogen belong to?