During fibrinolysis, tissue plasminogen activator (tPA) converts plasminogen to plasmin. Plasmin then degrades cross-linked fibrin. Which of the following correctly describes the action of plasmin?

• A Directly degrades fibrinogen but not cross-linked fibrin
• B Inhibits thrombin by binding to antithrombin III
• C Activates factor XIII to cross-link fibrin monomers
• D Cleaves fibrin into fibrin degradation products including D-dimers ✓

Explanation

Plasmin is a serine protease that degrades both fibrin and fibrinogen into soluble fragments. When it acts on cross-linked fibrin, it produces B-dimers, which are specific markers of fibrinolysis and are used clinically to diagnose venous thromboembolism. Plasmin does not activate factor XIII or inhibit thrombin; it breaks down fibrin clots, not builds them.

Reference: Hoffbrand's Postgraduate Haematology, 7th ed.

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