<prompt> In blood coagulation, the rate-limiting step is:Fibrinogen to fibrin conversion by action of thrombinConversion of prothrombin to thrombinActivation of factor XAction of factor VIII  <response>: 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. 