Which statement best describes heparin's mechanism?

• A. It inactivates thrombin and prevents conversion of fibrinogen to fibrin
• B. It directly inhibits platelets
• C. It activates plasmin
• D. It blocks tissue factor

Answer: (A)

Heparin works by binding to antithrombin III and dramatically accelerating its ability to inactivate thrombin (factor IIa) and factor Xa. When thrombin is inhibited, fibrinogen cannot be converted into fibrin, so clot formation is prevented. This direct effect on the coagulation cascade explains why the statement describing inactivation of thrombin and prevention of fibrin formation is the best description of heparin’s mechanism.

It doesn’t directly suppress platelets, so it’s not a platelet inhibitor. It doesn’t activate plasmin to dissolve clots (that would be plasminogen activators like tPA). And it doesn’t block tissue factor, which initiates the extrinsic pathway of coagulation.