Which inhibitor serves as a cofactor and accelerates the inactivation of Factor Va and Factor VIIIa?

The correct choice is Protein S, which plays a crucial role in the regulation of coagulation by acting as a cofactor for Protein C. When activated, Protein C inactivates Factors Va and VIIIa, which are essential for the clotting process. Protein S enhances the efficiency of this inactivation by binding to activated Protein C and the target cofactors, allowing for a more rapid reduction of thrombin generation and contributing to the overall regulation of the coagulation cascade. This mechanism is vital in maintaining hemostatic balance, preventing excessive clotting.

While the other options are important components of the hemostatic and anticoagulant systems, they do not share this specific cofactor role alongside Protein C in the context of Factor Va and VIIIa inactivation. Antithrombin III, for instance, primarily inhibits thrombin and Factor Xa, while alpha2-antiplasmin is involved in fibrinolysis, and does not interact with Factors Va and VIIIa in the same manner.