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
Causes of soft S1 include: 
Select and explain. 
A. Long standing severe MS, B. Obesity, C. Mitral Regurgitation, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Answer is D (All of the above) Long standing severe M.S. (calcified mitral valve), obesity, mitral regurgitation and pleural effusions are all established causes of soft S1.