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: Surgery
Syncope is usually caused by: 
Select and explain. 
A. Vasoconstriction, B. Cerebral ischemia, C. Cerebral hyperemia, D. Decrease in the vascular bed
B: Cerebral ischemia
Answer: B: Cerebral ischemia 
Explanation: None