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
Conversion of prekallikrein to kallikrein requires which clotting factor - 
Select and explain. 
A. XIII, B. XII, C. XI, D. X
B: XII
Answer: B: XII 
Explanation: Ans. is'b'i.e., XIIInitiation of intrinsic coagulation pathway occurs when factor XII is exposed to negatively charged surface.This leads to activation offactor XII to XIIa.Factor XIIa can then hydrolyze prekallikrein to kallikrein, which in turn activates more XII to XIIa.