Final answer:
Factor IX is activated in vivo by factor XIIa in the intrinsic pathway of the blood coagulation system, which can also be enhanced by factor VIIIa as part of a complex that leads to the activation of factor X and the common pathway.
Step-by-step explanation:
The student asked which factor can activate coagulation factor IX in vivo. The correct answer is b) Factor XIIa, which is part of the intrinsic pathway of blood coagulation. Activation of factor IX can also be enhanced by factor VIIIa, which forms a complex with factor IX to activate factor X, leading into the common pathway. However, the direct activation of factor IX in the intrinsic pathway is initiated by factor XIIa.
Intrinsic Pathway
The intrinsic pathway begins when factor XII (Hageman factor) is activated, which then sets off a cascade that activates factor XI and subsequently factor IX. Chemicals released by platelets accelerate these reactions. Activated factor IX forms a complex with factor VIII to further activate factor X. This step merges into the common pathway, where through a series of reactions, prothrombin is converted to thrombin, and fibrinogen is ultimately converted to fibrin to form a blood clot.