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Pharmacology · single question

A patient taking warfarin bleeds excessively. What is warfarin's mechanism of action that explains this?

  1. Direct thrombin inhibition
  2. Inhibition of platelet ADP receptors
  3. Vitamin K antagonism, reducing synthesis of clotting factors II, VII, IX, and X
  4. Activation of fibrinolysis
Show answer and explanation

Correct answer: C. Vitamin K antagonism, reducing synthesis of clotting factors II, VII, IX, and X

  • A. Incorrect. Direct thrombin inhibition is the MOA of dabigatran — not warfarin.
  • B. Incorrect. Platelet ADP receptor inhibition is clopidogrel's MOA. Warfarin does not affect platelets.
  • C. Correct. Warfarin is a VITAMIN K ANTAGONIST. It inhibits vitamin K epoxide reductase, reducing hepatic synthesis of vitamin K-dependent coagulation factors: II (prothrombin), VII, IX, X, and anticoagulant proteins C and S. Without these factors, the coagulation cascade is impaired → bleeding.
  • D. Incorrect. Fibrinolysis activation (dissolving clots) is the MOA of thrombolytics (tPA, streptokinase). Warfarin prevents NEW clot formation.

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