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 hea sound occurs during the period of: 
Select and explain. 
A. Isovolumetric relaxation, B. Isotonic relaxation, C. Isovolumetric contraction, D. Isotonic contraction
C: Isovolumetric contraction
Answer: C: Isovolumetric contraction 
Explanation: First hea sound is a low, slightly prolonged lub caused by vibrations set up by the sudden closure of mitral or tricuspid valves at the sta or ventricular systole and it occurs during the period of Isovolumetric contraction. Ref: Medical Physiology: Principles for Clinical Medicine, 3rd Ed, Page 247 By Rodney Rhoades, David R. Bell, Ph.D