This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Medicine
[{'content': 'Most common cause of spontaneous SAH \nSelect and explain. \nA. Trauma, B. Aneurysm, C. AV malformation, D. Hypeension', 'role': 'user'}, {'content': "Answer: B \nExplanation: *85%of SAH are caused by saccular or 'berry' aneurysms arising from the bifurcation of cerebral aeries paicularly in region of the circle of willis . Ref Harrison20th edition pg 2445", 'role': 'assistant'}]