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: Physiology
The second heart sound is caused by 
Select and explain. 
A. Closure of the aortic and pulmonary valves., B. Vibrations in the ventricular wall during systole., C. Ventricular filling., D. Closure of the mitral and tricuspid valves.
A: Closure of the aortic and pulmonary valves.
Answer: A: Closure of the aortic and pulmonary valves. 
Explanation: The second is a shorter, high-pitched “dup” (second sound), caused by vibrations associated with closure of the aortic and pulmonary valves just after the end of ventricular systole.