The extrinsic pathway of coagulation is physiologically initiated by exposure of tissue factor to blood. Tissue factor forms a complex with which coagulation factor to activate factor X?

• A Factor IX
• B Factor VII ✓
• C Factor XI
• D Factor V

Explanation

Tissue factor binds factor VII (or its activated form VIIa) to form the extrinsic tenase complex (TF-VIIa), which directly activates factor X to Xa and also activates factor IX. This complex is the primary physiological initiator of coagulation in vivo and is measured by the prothrombin time. Factor IX and XI are intrinsic pathway factors, and factor V is a cofactor, not a TF partner.

Reference: Hoffbrand's Postgraduate Haematology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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