Pulmonary Embolism USMLE Step 1 Practice Question
A 72-year-old man with a history of DVT 2 years ago (treated with 6 months of warfarin) presents with acute dyspnea. Clinical pretest probability for PE is high. D-dimer is 3.2 mcg/mL, and CTPA confirms PE. He is started on IV unfractionated heparin. Which of the following represents the mechanism of action of heparin in this case?
Answer choices
- ADirect inhibition of thrombin and factor Xa through antithrombin III activationCorrect answer
- BDirect conversion of plasminogen to plasmin for clot dissolution
- CInhibition of von Willebrand factor binding to glycoprotein Ib
- DIrreversible inhibition of platelet aggregation via COX-1 blockade
- EVitamin K-dependent inhibition of factors II, VII, IX, and X
- FHeparin directly inhibits factor Xa without requiring antithrombin III as a cofactor
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.