<prompt> Which of the following ion is needed for the conversion of prothrombin to thrombin?SodiumPotassiumMagnesiumCalcium  <response>: Prothrombin activator is formed as a result of rupture of a blood vessel or as a result of damage to special substances in the blood. Then the prothrombin activator, in the presence of sufficient amounts of ionic Ca, causes conversion of prothrombin to thrombin. Thrombin causes polymerization of fibrinogen molecules into fibrin fibers. Ref: Physiology Secrets By Hershel Raff, 2003, Page 66; Guytons physiology 22nd edition, Page 459. 