**Question:** Fibrin is degraded by-

A. Plasmin
B. Thrombin
C. Factor XIII
D. Fibrinolysin

**Core Concept:** Fibrin is a key component of the blood clotting cascade, formed from fibrinogen through the action of thrombin. Once the clot is no longer needed, it needs to be degraded to prevent excessive clot formation and maintain homeostasis. This process is known as fibrinolysis.

**Why the Correct Answer is Right:** Plasmin is an enzyme that plays a critical role in fibrinolysis. It is produced from plasminogen, an inactive precursor, through the action of tissue-type plasminogen activator (tPA) or urokinase (uPA). Plasmin degrades fibrin, thereby breaking down the clot and restoring normal blood flow.

**Why Each Wrong Option is Incorrect:**

B. Thrombin: Thrombin is the enzyme responsible for converting fibrinogen to fibrin during the clotting cascade. It is not involved in the degradation of fibrin.
C. Factor XIII: Factor XIII is a clotting factor and not an enzyme responsible for fibrin degradation. Its role is to cross-link fibrin strands, enhancing clot strength.
D. Fibrinolysin: Fibrinolysin is an incorrect term for plasmin, the enzyme responsible for fibrin degradation.

**Clinical Pearl:** Understanding fibrinolysis is essential for proper management of conditions associated with excessive clot formation or clot dissolution, such as deep vein thrombosis, pulmonary embolism, and heparin-induced thrombocytopenia. Adequate knowledge of clotting factors and their regulation allows for targeted treatment strategies.