This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': 'Conversion of Prothrombin to thrombin requires \nSelect and explain. \nA. Ca++, B. Va, Ca++, C. V, X, Ca++, D. X, V, XII, & Ca++', 'role': 'user'}, {'content': 'Answer: B \nExplanation: In the presence of tissue phospholipids (PL) and calcium ions, activated Factor X (Xa) combines with Factor Va to form prothrombin activator which initiates the conversion of prothrombin to thrombin.Factor V, X, and XII are activated and involved in other steps of intrinsic pathway.Ref: Ganong&;s review of medical physiology, 23rd edition, page: 533', 'role': 'assistant'}]