**Core Concept**
Isovolumetric relaxation is a phase of the cardiac cycle where the ventricles relax and decrease in volume without any change in volume or pressure. It occurs after the isovolumetric contraction phase and before the ventricular ejection phase. This phase is essential for the proper functioning of the heart, allowing it to refill with blood and prepare for the next contraction.

**Why the Correct Answer is Right**
During isovolumetric relaxation, the ventricles relax and the pressure inside them decreases. At the same time, the aortic and pulmonary valves remain closed due to the higher pressure in the aorta and pulmonary artery. As the ventricular pressure falls below the pressure in the great vessels, the mitral and tricuspid valves open, allowing blood to flow into the ventricles. This phase is characterized by a rapid decrease in ventricular pressure, which is mediated by the relaxation of the cardiac muscle and the opening of the ventricular myocytes' potassium channels, allowing potassium ions to flow out of the cell.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Isovolumetric relaxation is an essential phase of the cardiac cycle, and any abnormalities in this phase can lead to conditions such as diastolic dysfunction, which can result in conditions like heart failure with preserved ejection fraction (HFpEF).

**Correct Answer:** C.