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
Prothrombin to thrombin conversion need 
Select and explain. 
A. Calcium, B. Magnesium, C. Potassium, D. Sodium
A: Calcium
Answer: A: Calcium 
Explanation: Calcium needed in all the steps of the clotting EXCEPT: first two steps of intrinsic pathway under stimulatory effect of HMW kininogen & prekallikren