What is plasmin's role in fibrinolysis and which marker indicates its activity?

• A. Plasmin degrades fibrin; D-dimer is a marker of cross-linked fibrin degradation.
• B. Plasmin promotes fibrin formation; D-dimer decreases.
• C. Plasmin degrades platelets; D-dimer is platelet-derived.
• D. Plasmin is not involved in fibrinolysis; D-dimer marker indicates clot presence.

Answer: (A)

Plasmin’s role in fibrinolysis is to break down the fibrin mesh that stabilizes a clot, effectively dissolving the clot. As plasmin digests cross-linked fibrin, it generates degradation products, including D-dimer. Because D-dimer arises specifically from the breakdown of cross-linked fibrin, its presence in the blood signals active fibrinolysis driven by plasmin and recent clot dissolution. The other statements misstate plasmin’s function or what D-dimer represents: plasmin does not promote fibrin formation, it does not primarily degrade platelets, and D-dimer is a product of fibrin degradation rather than a general marker of clot presence alone.