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': 'Which physical sign characterizes mitral stenosis: \nSelect and explain. \nA. Loud first heart sound, B. Soft single second heart sound, C. Third heart sound, D. Pulsatile liver', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]