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

The extrinsic pathway of coagulation is primarily initiated by tissue factor (also known as factor 3) that is exposed at the site of blood vessel injury. This pathway also involves factor 7, which, once activated (to factor 7a), complexes with tissue factor and activates factor 10. The extrinsic pathway is crucial for rapid hemostatic response to vascular injuries.

Factor 10, although part of the coagulation cascade, plays a role downstream of the extrinsic pathway, specifically in the common pathway where it is activated by factor 7a in the presence of tissue factor and subsequently leads to the conversion of prothrombin to thrombin.

Factor 5 does not participate directly in the extrinsic pathway of coagulation. Instead, it acts later in the process by forming a complex with factor 10a (after it has been activated) to convert prothrombin to thrombin in the common pathway.

Thus, factor 5 is not involved in the extrinsic pathway, and therefore it is the correct response to the question regarding which factor does not participate in this specific pathway.