To which pathway does Factor X belong in the described activation sequence?

• A. Common pathway
• B. Intrinsic pathway
• C. Extrinsic pathway
• D. Fibrinolytic pathway

Answer: (A)

Factor X sits at the point where the two initiation pathways converge, marking the start of the common pathway. It is activated to Factor Xa by both the intrinsic tenase complex (IXa-VIIIa-Ca2+) and the extrinsic tenase complex (VIIa with tissue factor). Once Xa is formed, it combines with Va to convert prothrombin to thrombin, which then drives the formation of fibrin and the clot. The fibrinolytic pathway, in contrast, breaks down fibrin, not involved in activating Factor X. So Factor X belongs to the common pathway because its activation represents the shared route from initiation to the generation of thrombin and fibrin.