Which clotting factor acts as an enzyme to convert soluble fibrinogen into insoluble fibrin threads during blood coagulation?

Answer and explanation

Correct answer: A. Thrombin (Factor IIa)

  1. Step 1: Hemostasis culminates in a proteolytic enzyme cascade that transforms soluble blood proteins into a durable polymeric clot.
  2. Step 2: Prothrombin activator converts the zymogen prothrombin (Factor II) into active thrombin (Factor IIa).
  3. Step 3: Thrombin then hydrolyzes fibrinopeptides from soluble fibrinogen (Factor I), allowing fibrin monomers to rapidly cross-link into an insoluble fibrin meshwork.
Answer
Thrombin (Factor IIa).
Why not the others: Prothrombin is the inactive zymogen precursor of thrombin synthesized by the liver; Thromboplastin (tissue factor, Factor III) initiates the extrinsic coagulation pathway by activating Factor VII; Calcium ions (Factor IV) alone serve as an essential inorganic cofactor across multiple enzymatic steps but do not possess direct proteolytic activity on fibrinogen.

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