After starting warfarin therapy, which vitamin K-dependent coagulation factor declines first and accounts for the early prolongation of prothrombin time?

• A Factor II
• B Factor IX
• C Factor X
• D Factor VII ✓

Explanation

Warfarin blocks vitamin K epoxide reductase, preventing gamma-carboxylation of factors II, VII, IX, and X and proteins C and S. Factor VII has the shortest half-life of this group, approximately 6 hours, so its level falls first and produces the earliest rise in prothrombin time (INR). Prothrombin, with a half-life of around 60 to 72 hours, declines much later, which is why full anticoagulation requires several days despite a rising INR within 24 to 48 hours.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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