CNS Pharmacology USMLE Step 1 Practice Question
A 72-year-old man on warfarin 5 mg daily for atrial fibrillation develops community-acquired pneumonia and is prescribed erythromycin 500 mg four times daily. Three days later, he presents with gingival bleeding and hematuria. Vital signs: BP 128/76, HR 92, RR 18, Temp 37.2°C, SpO2 98%. Laboratory findings show INR 9.2 (previously 2.8), hemoglobin 10.2 g/dL (baseline 13.5), and PT 68 seconds. Physical exam reveals scattered gingival oozing and bilateral crackles on auscultation. Chest X-ray confirms right lower lobe pneumonic infiltrate. Which mechanism best explains this drug interaction?
Answer choices
- ADecreased warfarin metabolism due to cytochrome P450 inhibitionCorrect answer
- BIrreversible inhibition of platelet cyclooxygenase
- CEnhanced renal elimination of warfarin by urine alkalinization
- DReduced warfarin bioavailability from first-pass metabolism
- EA pharmacodynamic decrease in efficacy caused by receptor downregulation
- FDisplacement of warfarin from plasma protein binding by erythromycin
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