**Core Concept**
Isovolumic relaxation phase is a critical component of the cardiac cycle, characterized by the rapid decline in ventricular pressure during diastole, without any change in ventricular volume. This phase is essential for the transition from systole to diastole, ensuring a smooth and efficient cardiac cycle.

**Why the Correct Answer is Right**
During isovolumic relaxation, the ventricles relax and the pressure inside the ventricles decreases rapidly. This is due to the decrease in contractile state of the myocardium, which is mediated by the relaxation of myosin heads and the decrease in intracellular calcium levels. The decrease in ventricular pressure eventually leads to the opening of the mitral and tricuspid valves, marking the end of the isovolumic relaxation phase. The isovolumic relaxation phase is crucial for the proper functioning of the cardiac cycle, as it ensures that the ventricles relax and fill with blood before the next contraction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the isovolumic relaxation phase does not involve the opening of the aortic and pulmonary valves, which occur during the early rapid filling phase.
* **Option B:** This option is incorrect because the isovolumic relaxation phase does not involve the closure of the mitral and tricuspid valves, which occurs during the late rapid filling phase.
* **Option D:** This option is incorrect because the isovolumic relaxation phase does not involve the contraction of the myocardium, which occurs during systole.

**Clinical Pearl / High-Yield Fact**
The isovolumic relaxation phase is often referred to as the "rapid filling phase" because of the rapid decrease in ventricular pressure, which allows for the rapid filling of the ventricles with blood.

**Correct Answer: C. Opening of the mitral and tricuspid valves.**