Which factor is not involved in the extrinsic pathway of coagulation?

The extrinsic pathway of coagulation is primarily initiated by the exposure of tissue factor (also known as thromboplastin or factor III) to circulating factor VII when there is vascular injury. This pathway is crucial for the rapid activation of the coagulation cascade, leading to the formation of a stable blood clot.

Factor VII is essential in this pathway as it complexes with tissue factor (factor III) to activate factor X. Factor III plays a significant role as it is the tissue factor that initiates the coagulation process. Factor X becomes activated to Xa, which then plays a central role further in the common pathway of coagulation.

Factor V, on the other hand, is part of the common pathway and acts as a cofactor in the conversion of prothrombin to thrombin, but it is not part of the extrinsic pathway itself. Therefore, identifying factors that specifically belong to the extrinsic pathway clarifies the role of each factor in hemocoagulation.

Thus, among the options given, factor V is not involved in the extrinsic pathway, making it the correct choice in this context. Its participation occurs later during clot formation, emphasizing the distinction between intrinsic (factors present in blood) and extrinsic (factors from damaged