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
Loud S1 in mitral stenosis is seen in 
Select and explain. 
A. Prolonged flow through mitral valve, B. 1st degree heart block, C. Calcification of the valve, D. Immobilization of the valve
A: Prolonged flow through mitral valve
Answer: A: Prolonged flow through mitral valve 
Explanation: None