Which anticoagulant is known to inhibit fibrin directly?

• A. Plasmin
• B. Protein S
• C. Tissue factor pathway inhibitor
• D. Z inhibitor protein (ZIP)

Answer: (A)

The correct understanding lies in recognizing that plasmin is an enzyme that plays a crucial role in the fibrinolysis process, which is the breakdown of fibrin in blood clots. Plasminogen is converted into plasmin, which then acts directly on fibrin, leading to its cleavage and subsequent dissolution of clots. This process is essential for maintaining normal blood flow and preventing excessive clot formation.

The other options involve regulatory roles in coagulation but do not directly inhibit fibrin. Protein S is a vitamin K-dependent plasma protein that acts as a cofactor to Protein C in inactivating factors Va and VIIIa. The tissue factor pathway inhibitor serves to regulate the coagulation cascade by inhibiting factor Xa and factor VIIa, but it does not have a direct action on fibrin itself. Z inhibitor protein (ZIP) is also more about the regulation of coagulation factors rather than directly cleaving fibrin. Therefore, the key function of plasmin as an active agent in fibrin degradation makes it the correct choice.