This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiac Cycle, Subject name: Physiology
Which phase of the cardiac cycle follows immediately after the beginning of the QRS wave? 
Select and explain. 
A. Isovolumic relaxation, B. Atrial systole, C. Diastasis, D. Isovolumic contraction
D: Isovolumic contraction
Answer: D: Isovolumic contraction 
Explanation: Immediately after the QRS wave, the ventricles begin to contract, and the first phase that occurs is isovolumic contraction. Isovolumic contraction occurs before the ejection phase and increases the ventricular pressure enough to mechanically open the aoic and pulmonary valves.