NBME Microbiology 3

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?

Accepted Answer

Paramyxoviridae

Answer

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?

Accepted Answer

Reverse transcriptase

Answer

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?

Accepted Answer

Toxoplasma gondii

Answer

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?

Accepted Answer

Hepatitis B virus

Answer

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?

Accepted Answer

B lymphocytes

Answer

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.

NBME Practice Test

NBME Practice Test

NBME Microbiology 3