This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood: Hemostasis and Blood Coagulation, Subject name: Physiology
Factor which activates prekallikrein - 
Select and explain. 
A. Factor VIII, B. Factor II, C. Factor X, D. Factor XII
D: Factor XII
Answer: D: Factor XII 
Explanation: Ans. is 'd' i.e., Factor XIIo Initiation of intrinsic coagulation pathway occurs when factor XII is exposed to negatively charged surfaceo This leads to activation of factor XII to XIIa.o Factor XIIa can then hydrolyze prekallikrein to kallikrein, which in turn activates more XII to XIIa.