Blood coagulation is initiated by either the intrinsic or extrinsic pathway. The final step in both pathways would be:
A) Activation of factor XII
B) Deactivation of fibrinogen, preventing the conversion to fibrin
C) Activation of factor X, leading to conversion of prothrombin II to thrombin IIa
D) Activation of factors VII, X, V, and II
Ans: C
Feedback:
The terminal steps in both pathways are the same: the activation of factor X and the conversion of prothrombin (II) to thrombin (IIa). Thrombin then acts as the enzyme to convert fibrinogen (I) to fibrin (Ia), the material that stabilizes a clot. Factor XII initiates the intrinsic pathway in circulation, whereas the extrinsic pathway is initiated by tissue factor when tissues are injured. Protein C acts as an anticoagulant by inactivating factors V and VIII. Thrombin is formed during the coagulation process and does initiate it.
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