Question

The extrinsic system of coagulation is activated by?

• A. Factor XI
• B. Factor X
• C. Factor XII
• D. Factor III

Hint:

The pathway from outside the vessel begins with tissue factor.

Answer 1

The Correct Option is D

Step 1: The extrinsic pathway of coagulation is triggered when blood vessels are damaged and tissue is exposed. The trigger molecule is tissue factor, also called tissue thromboplastin or Factor III.

Step 2: Factor III combines with Factor VII and calcium to activate Factor X, which starts the common pathway leading to thrombin and fibrin. So the activator of the extrinsic system is Factor III.

Step 3: Factor XII (option c) and Factor XI (option a) belong to the intrinsic pathway, which is set off by contact with a negatively charged surface. Factor X (option b) is the point where the two pathways converge, not the initiator of the extrinsic limb.

Step 4: Therefore the extrinsic pathway is activated by tissue factor, Factor III, making option d correct.