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 caused by: 
Select and explain. 
A. Prolonged flow through mitral valve, B. 1st degree hea block, C. Calcification of the valve, D. Immobilization of valve
A: Prolonged flow through mitral valve
Answer: A: Prolonged flow through mitral valve 
Explanation: Answer is A (Prolonged flow through mitral valve) Loud SI in mitral stenosis results from prolonged AV flow through the mitral vain `S1 is louder i fAV flow is prolonged because of mitral stenosis' -- Harrison Reduced mobility of valve (Immobility, calcification), and Prolonged PR interval (1st degree block) are all causes of soft S1. Note: First degree block is characterized by prolonged PR interval.