Which factor is responsible for covalently linking fibrin monomers into fibrin polymers?

The correct answer is Factor XIII, which plays a crucial role in the coagulation cascade. After thrombin converts fibrinogen to fibrin monomers during the clotting process, Factor XIII catalyzes the formation of covalent cross-links between the fibrin monomers. This cross-linking is essential for stabilizing the fibrin clot and ensuring its structural integrity, which is vital for proper hemostasis.

Factor XIII is activated by thrombin, and once activated, it forms cross-links between adjacent fibrin strands, effectively converting soluble fibrin into a solid and stable fibrin mesh that holds the clot together. This stabilization is critical for wound healing and preventing excessive bleeding.

Other factors listed—such as Factors V, VII, and VIII—have different roles in the coagulation pathway. For instance, Factor V acts as a cofactor in the prothrombinase complex, Factor VII is involved in the extrinsic pathway of coagulation, and Factor VIII works as a cofactor for Factor IX in the intrinsic pathway. Each has important functions in promoting clot formation, but they do not directly contribute to the covalent linking of fibrin monomers, which is the specific action of Factor XIII.