Which molecule is activated by thrombin in the presence of thrombomodulin to help limit clot formation?

• A. Factor V
• B. Factor VIII
• C. Protein C
• D. Tissue factor

Answer: (C)

Thrombin bound to thrombomodulin on endothelial cells shifts its role from driving clot formation to activating an anticoagulant pathway. This complex specifically activates protein C to activated protein C (APC). APC, with its cofactor protein S, then inactivates Factors Va and VIIIa, which are essential for accelerating thrombin generation. By disabling these cofactors, the amplification of the coagulation cascade is dampened, helping to limit clot formation.

Factor V and Factor VIII are coagulation cofactors that thrombin can activate to promote clotting, so they don’t serve to limit clot formation in this context. Tissue factor initiates the extrinsic pathway with factor VIIa, rather than being activated by thrombin in the thrombomodulin pathway.