Virology USMLE Step 1 Practice Question
A 6-year-old previously unvaccinated boy presents to the clinic with a 4-day history of fever (39.5°C), cough, coryza, and conjunctivitis. On examination, you note white spots with red halos on the buccal mucosa opposite the molars. Today, a faint maculopapular rash appeared on his hairline and behind his ears. His mother reports that his symptoms began 2 days after exposure to a classmate with similar illness. Serum measles IgM is positive and IgG is negative. Which of the following best explains why the characteristic measles rash typically emerges 3-4 days after the onset of fever and respiratory symptoms?
Answer choices
- AThe rash represents direct cytopathic damage from rapid viral replication within dermal keratinocytes and fibroblasts
- BThe rash coincides with peak measles viremia and maximal IgM antibody production in secondary lymphoid tissues
- CThe rash appears as IgG antibody titers rise and neutralizing antibodies clear virus from the bloodstream, triggering immune complex deposition in skin
- DThe rash emerges during declining viremia as cell-mediated immunity develops and IgG antibodies begin to neutralize circulating virusCorrect answer
- EThe rash is an early manifestation of viral dissemination before antibody production occurs, causing direct endothelial infection
- FThe rash results from bacterial superinfection of skin lesions during the prodromal phase of measles
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