Final answer:
The critical step in blood clot formation is the conversion of fibrinogen to fibrin during the coagulation process, facilitated by the common pathway after activation by intrinsic or extrinsic pathways.
Step-by-step explanation:
The conversion of fibrinogen to fibrin is the critical step in the formation of a blood clot. During the coagulation process, this conversion forms a mesh that traps platelets and erythrocytes, leading to the clot that prevents further bleeding. Both the intrinsic and extrinsic pathways lead to the common pathway, wherein factor X is activated, leading to the conversion of prothrombin into thrombin, which, in turn, converts fibrinogen into fibrin, thereby producing the fibrin mesh.