Activation of the protein C pathway by thrombin–thrombomodulin serves to inactivate which factors?

• A. Inactivates factors Va and VIIIa
• B. Activates factor VII
• C. Converts prothrombin to thrombin
• D. Degrades fibrin

Answer: (A)

Thrombin–thrombomodulin switches thrombin from a procoagulant to an anticoagulant function by activating protein C. Activated protein C, with its cofactor protein S, proteolytically inactivates factors Va and VIIIa. Va is the cofactor in the prothrombinase complex that converts prothrombin to more thrombin, and VIIIa is the cofactor in the tenase complex that helps activate factor X. By removing these cofactors, the generation of thrombin is dampened and clotting is downregulated. The other options don’t fit because APC does not activate factor VII, does not directly convert prothrombin to thrombin, and does not degrade fibrin (that is the job of plasmin).