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
First heart sound corresponds to: 
Select and explain. 
A. Mitral valve opening, B. Mitral valve closing, C. Aortic valve closing, D. None of the above
B: Mitral valve closing
Answer: B: Mitral valve closing 
Explanation: The first heart sound (S1) includes mitral and tricuspid valve closure. 

The First heart sound is associated with closure of the A-V Valves.
The Second heart sound Is associated with closure of the Aortic and Pulmonary Valves.
The Third heart sound occurs at the beginning of the middle third of diastole. Occasionally a weak, rumbling third heart sound is heard at the beginning of the middle third of diastole.  
Atrial contraction sound (Fourth heart sound). 

Reference: : Guyton and Hall Physiology page no  283,284