This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Which of the following ion is needed for the conversion of prothrombin to thrombin? \nSelect and explain. \nA. Sodium, B. Potassium, C. Magnesium, D. Calcium', 'role': 'user'}, {'content': "Answer: D \nExplanation: 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; Guyton's physiology 22nd edition, Page 459.", 'role': 'assistant'}]