Unfractionated heparin produces its anticoagulant effect by binding antithrombin III. Which factor is inhibited LEAST efficiently by this complex?

• A Factor VIIa ✓
• B Factor Xa
• C Factor IIa (thrombin)
• D Factor XIIa

Explanation

The heparin-antithrombin complex potently inhibits thrombin (IIa) and Xa, and also XIIa, XIa and IXa. Factor VIIa is a poor substrate for antithrombin inhibition because it circulates largely bound to tissue factor, where the extrinsic pathway operates on cell surfaces beyond reach of circulating complexes. This is why heparin monitoring uses aPTT, which reflects intrinsic pathway factors, rather than PT.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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