Cardiovascular Drugs USMLE Step 1 Practice Question
A 65-year-old woman with atrial fibrillation presents to her cardiologist for rate control. She has been stable on warfarin (target INR 2-3) for 6 months for stroke prevention. Her baseline INR was 2.4 one month ago. The cardiologist initiates diltiazem 120 mg daily for rate control due to recurrent palpitations and a resting heart rate of 108 bpm. Five days later, the patient returns for routine INR monitoring. Laboratory results show INR 4.8 (PT 42 seconds, control 12 seconds). The patient denies any signs of bleeding, and her vital signs are stable. Which of the following best explains the elevated INR in this patient?
Answer choices
- ADiltiazem competitively inhibits CYP2C9, the primary isoenzyme responsible for S-warfarin metabolismCorrect answer
- BDiltiazem induces hepatic metabolism of warfarin through upregulation of CYP3A4
- CDiltiazem displaces warfarin from plasma protein binding, increasing the free fraction of warfarin
- DDiltiazem inhibits vitamin K epoxide reductase, potentiating warfarin's anticoagulant effect
- EDiltiazem reduces intestinal absorption of warfarin, leading to compensatory increased metabolism
- FDiltiazem undergoes extensive hepatic metabolism via CYP3A4, competing with warfarin for enzyme access and reducing warfarin clearance
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