This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood Physiology, Subject name: Physiology
In blood coagulation, the rate-limiting step is: 
Select and explain. 
A. Fibrinogen to fibrin conversion by action of thrombin, B. Conversion of prothrombin to thrombin, C. Activation of factor X, D. Action of factor VIII
C: Activation of factor X
Answer: C: Activation of factor X 
Explanation: Activation of factor X or formation of prothrombin activator (Xa) is the rate-limiting step in the clotting cascade. Extrinsic pathway for coagulation: Tissue trauma --> release of tissue thromboplastin --> eventual conversion of X to Xa. Intrinsic pathway for coagulation: Trauma to blood, or blood (factor XII) coming in contact with subendothelial collagen --> other clotting factors activated in the subsequent cascade --> eventual conversion of X to Xa.