Pulmonary Embolism USMLE Step 1 Practice Question
A 42-year-old woman with a history of provoked pulmonary embolism 8 months ago presents to the emergency department with acute onset dyspnea and hemoptysis. She has been on warfarin maintenance therapy since her prior PE. Vital signs: temperature 37.2°C, heart rate 118/min, blood pressure 128/82 mmHg, respiratory rate 22/min, oxygen saturation 88% on room air. Physical examination reveals tachypnea and decreased breath sounds at the right lung base. Laboratory studies show INR 6.8, troponin 0.02 ng/mL (normal <0.04), and D-dimer >500 ng/mL. CT pulmonary angiography confirms acute pulmonary embolism with a right lower lobe subsegmental filling defect. The patient is started on intravenous unfractionated heparin, and warfarin is held. Which of the following best explains the clinical rationale for initiating heparin rather than increasing the warfarin dose in the acute management of this PE?
Answer choices
- AHeparin has superior bioavailability compared to warfarin due to its ability to cross the blood-brain barrier
- BHeparin can be rapidly reversed with protamine sulfate, whereas warfarin reversal requires fresh frozen plasma and vitamin K
- CHeparin achieves therapeutic anticoagulation within hours by potentiating antithrombin III, whereas warfarin requires 5-7 days to achieve full therapeutic effect by depleting vitamin K-dependent factorsCorrect answer
- DHeparin directly lyses existing thrombi through activation of plasminogen, while warfarin only prevents new clot formation
- EWarfarin is contraindicated in patients with elevated INR values above 6.0 due to increased risk of spontaneous bleeding
- FHeparin has a shorter half-life, allowing more rapid dose adjustments if bleeding complications develop
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