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
Conversion of Prothrombin to thrombin requires 
Select and explain. 
A. Ca++, B. Va, Ca++, C. V, X, Ca++, D. X, V, XII, & Ca++
B: Va, Ca++
Answer: B: Va, Ca++ 
Explanation: 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