**Core Concept**
Isovolumic contraction phase is the initial phase of ventricular systole where the ventricular pressure increases without any change in volume, resulting in a rapid increase in intraventricular pressure. This phase is characterized by the closure of the mitral and tricuspid valves and the opening of the aortic and pulmonary valves.

**Why the Correct Answer is Right**
During the isovolumic contraction phase, the ventricular muscle contracts and increases the pressure inside the ventricle. As a result, the pressure inside the ventricle exceeds the pressure in the aorta and pulmonary artery. However, the aortic and pulmonary valves are still closed due to the lower pressure in these vessels. Therefore, the ventricular volume does not change, and this phase is known as isovolumic contraction. The ECG marker that correlates with the isovolumic contraction phase is the initial upstroke of the QRS complex.

**Why Each Wrong Option is Incorrect**
**Option A:** The isovolumic relaxation phase occurs during diastole, when the ventricular pressure decreases and the aortic and pulmonary valves are closed.
**Option B:** The rapid ejection phase occurs during the early part of systole, when the aortic and pulmonary valves are open and the ventricular pressure is higher than the pressure in these vessels.
**Option C:** The isovolumic contraction phase is characterized by a rapid increase in intraventricular pressure, not a decrease.

**Clinical Pearl / High-Yield Fact**
The isovolumic contraction phase is an important phase of the cardiac cycle, and understanding its timing and characteristics is crucial for interpreting ECGs and diagnosing cardiac arrhythmias.

**Correct Answer:** C.