Which statement best describes protein C?

Which statement best describes protein C?

• A. Vitamin K-dependent inhibitor to clotting
• B. Activator of factors V and VIII:C
• C. Inhibitor of fibrinolysis
• D. Synthesized by endothelial cells

Answer: (A)

Protein C is a vitamin K–dependent anticoagulant protein produced by the liver. On the endothelial surface, thrombin bound to thrombomodulin activates protein C, and the activated form (with protein S as a cofactor) inactivates factors Va and VIIIa, reducing thrombin generation and curbing clot formation. That makes it an inhibitor of clotting. It does not activate Va or VIII:C; it does not primarily inhibit fibrinolysis; and it is produced by the liver, not the endothelium. Therefore, the best description is that protein C is a vitamin K–dependent inhibitor to clotting.

Protein C is a vitamin K–dependent anticoagulant protein produced by the liver. On the endothelial surface, thrombin bound to thrombomodulin activates protein C, and the activated form (with protein S as a cofactor) inactivates factors Va and VIIIa, reducing thrombin generation and curbing clot formation. That makes it an inhibitor of clotting. It does not activate Va or VIII:C; it does not primarily inhibit fibrinolysis; and it is produced by the liver, not the endothelium. Therefore, the best description is that protein C is a vitamin K–dependent inhibitor to clotting.