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': 'Pansystolic murmur at apex with soft first heart sound is the key sign of which one of the following ? \nSelect and explain. \nA. Aortic regurgitation, B. Mitral regurgitation, C. Ventricular septal defect, D. Atrial septal defect', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]