The answer that best completes the statement above is this: Activation of factor X, leading to conversion of prothrombin II to thrombin IIa. This is the final step in both pathways. The thrombin, which is an enzyme in the blood plasma, does the conversion of fibrinogen (I) to fibrin (Ia). This is the one responsible in clotting. The intrinsic pathway is initiated by Factor XII. On the other hand, tissue factor during injury initiates the extrinsic pathway.