This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: FMGE 2018, Subject name: Physiology
[{'content': 'Prothrombin to thrombin conversion need \nSelect and explain. \nA. Calcium, B. Magnesium, C. Potassium, D. Sodium', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Calcium needed in all the steps of the clotting EXCEPT: first two steps of intrinsic pathway under stimulatory effect of HMW kininogen & prekallikren', 'role': 'assistant'}]