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': 'Causes of soft S1 include: \nSelect and explain. \nA. Long standing severe MS, B. Obesity, C. Mitral Regurgitation, D. All of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Answer is D (All of the above) Long standing severe M.S. (calcified mitral valve), obesity, mitral regurgitation and pleural effusions are all established causes of soft S1.', 'role': 'assistant'}]