This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
[{'content': 'Atrial fibrillation may occur in all the following conditions except \nSelect and explain. \nA. Mitral stenosis, B. Hypothyroidism, C. Dilated cardiomyopathy, D. Mitral regurgitation', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Cardioversion within 48 h of the onset of AF is common practice in patients who have not been anticoagulated, provided that they are not at high risk for stroke due to a prior history of embolic events, rheumatic mitral stenosis, or hyperophic cardiomyopathy with marked left atrial enlargement. Ref Harrison 20th edition pg 1432', 'role': 'assistant'}]