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: Medicine
[{'content': 'Commonest cause of subarachnoid haemorrhage is: \nSelect and explain. \nA. Rupture of circle of Willis aneurysm, B. Rupture or veebral aery aneurysm, C. Rupture of venecomitants of corpus striatum, D. Rupture of dural sinusis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Answer is A (Rupture of circle of Willis aneurysm): Most common cause of subarachnoid haemorrhage is ruptured saccular aneurysms. Most common site of saccular aneurysms is the anterior circulation on the circle of Willis', 'role': 'assistant'}]