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
First heart sound occurs during the: 
Select and explain. 
A. Isovolumetric ventricular systole., B. Isovolumetric ventricular diastole., C. Protodisatole., D. Ventricular ejection.
A: Isovolumetric ventricular systole.
Answer: A: Isovolumetric ventricular systole. 
Explanation: Isovolumetric ventricular systole is ended with the opening of the seminar valves, due to which first heart sound is produced.