American Society of Hematology (ASH) ITE Practice Test

In fibrinolysis, plasmin breaks down fibrin clots, and its activity is tightly controlled by inhibitors that block both its formation and its action. PAI-1 is the main inhibitor that prevents plasminogen from being converted to plasmin by tissue-type and urokinase-type plasminogen activators. Alpha-2 antiplasmin directly binds plasmin and stops it from proteolyzing fibrin. Together, these two inhibitors suppress both the generation and the activity of plasmin, making them the inhibitors of plasmin and plasminogen.

The other options are mostly regulators of coagulation rather than fibrinolysis. Antithrombin and Protein C inhibit coagulation factors; TFPI inhibits the tissue factor–initiated coagulation pathway; thrombomodulin modulates thrombin activity; fibrinogen is a clotting substrate.