Virology USMLE Step 1 Practice Question
A 28-year-old woman with HIV infection (CD4 count 150 cells/μL) develops recurrent painful oral ulcers and is found to have esophageal ulcers on endoscopy. She is already on acyclovir but continues to worsen. A viral culture grows a thymidine kinase-deficient mutant. Which of the following mechanisms best explains the clinical failure?
Answer choices
- AThe virus is producing increased levels of viral glycoproteins
- BThe drug failed to cross the blood-brain barrier
- CThe patient has developed neutralizing antibodies to herpes simplex virus
- DThe virus has developed altered DNA polymerase with reduced drug binding
- EThe virus cannot phosphorylate acyclovir, preventing its incorporation into viral DNACorrect answer
- FThe virus has upregulated efflux pump expression, leading to increased drug expulsion from infected cells
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