Final answer:
In the common pathway of blood coagulation, prothrombin (factor II) is converted to thrombin by prothrombinase, leading to the formation of a fibrin clot.
Step-by-step explanation:
In the common pathway of blood coagulation, an inactive glycoprotein called prothrombin, or factor II, is converted to thrombin in the presence of calcium and phospholipids. This conversion is catalyzed by the enzyme prothrombinase, which has been activated by either the intrinsic or extrinsic pathway. Thrombin then acts to convert factor I, the soluble fibrinogen, into insoluble fibrin protein strands. Lastly, factor XIII stabilizes the fibrin clot, which is critical for the cessation of bleeding.