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 fourth 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.
C: Ventricular filling.
Answer: C: Ventricular filling. 
Explanation: A fourth sound can sometimes be heard immediately before the first sound when atrial pressure is high or the ventricle is stiff in conditions such as ventricular hypertrophy. It is due to ventricular filling and is rarely heard in normal adults.