**Core Concept**
The cardiac cycle is a sequence of events that occur in the heart from one heartbeat to the next. Ventricular systole is the phase of the cardiac cycle where the ventricles contract and pump blood out of the heart. The timing of ventricular systole is crucial for understanding the cardiac cycle and diagnosing cardiac abnormalities.

**Why the Correct Answer is Right**
The beginning of ventricular systole is marked by the closure of the mitral and tricuspid valves, which occurs at the start of the QRS complex on an electrocardiogram (ECG). This is due to the rapid depolarization of the ventricles, which causes the ventricular muscle to contract. The closure of the mitral and tricuspid valves prevents backflow of blood into the atria and allows the ventricles to pump blood out of the heart.

**Why Each Wrong Option is Incorrect**
* **Option A:** This point on the diagram does not mark the beginning of ventricular systole, as it occurs before the QRS complex and the closure of the mitral and tricuspid valves.
* **Option B:** This point on the diagram marks the end of diastole and the beginning of isovolumetric contraction, which is a part of the ventricular systolic phase but not the beginning.
* **Option D:** This point on the diagram marks the peak of ventricular systole, where the ventricles are fully contracted and the aortic and pulmonary valves are open, allowing blood to be pumped out of the heart.

**Clinical Pearl / High-Yield Fact**
The QRS complex on an ECG represents the period of rapid ventricular depolarization, which is responsible for the contraction of the ventricles and the pumping of blood out of the heart. This complex is a critical component of the cardiac cycle and is often used as a diagnostic tool in cardiac electrophysiology.

**Correct Answer:** C.