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': 'Loud S1 in mitral stenosis is seen in - \nSelect and explain. \nA. Prolonged flow through mitral valve, B. 1* degree hea block, C. Calcification of the valve, D. Immobilization of the valve', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In MS, blood filling of ventricles is slow, So leaflets remain open / far apa when impulse for ventricular contraction reaches. So loud closure occurs. Ref Harrison 20th edition pg 1456', 'role': 'assistant'}]