The main regulatory protein of secondary hemostasis is

• A. Antithrombin
• B. Protein C
• C. Alpha2-Antiplasmin
• D. Tissue plasminogen activator

Answer: (A)

Antithrombin acts as the main regulator of secondary hemostasis because it directly neutralizes thrombin and other coagulation serine proteases in the cascade, preventing excessive clot formation after injury. Its inhibitory effect is greatly enhanced by heparin, which is why heparin anticoagulants are so effective in stopping thrombosis. The other proteins listed participate in anticoagulation or fibrinolysis but do not serve as the principal brake on thrombin generation: alpha2-antiplasmin mainly inhibits plasmin to modulate fibrinolysis, tissue plasminogen activator promotes fibrinolysis, and the protein C system modulates with Va and VIIIa, but it is not the primary regulator of the coagulation cascade. Therefore, antithrombin is the key regulator of secondary hemostasis.