**Core Concept**
Isovolumetric relaxation phase is a critical period in the cardiac cycle where the ventricles relax and decrease in volume without any change in pressure. This phase is essential for the heart to refill with blood and prepare for the next contraction.

**Why the Correct Answer is Right**
During isovolumetric relaxation, the ventricles rapidly decrease in volume due to the closure of the aortic and pulmonary valves, which prevents blood from escaping. The pressure inside the ventricles falls below the pressure in the atria, allowing the atrioventricular (AV) valves to open. This transition marks the end of the isovolumetric relaxation phase. The key mechanism involves the relaxation of the cardiac myocytes, which reduces the ventricular pressure and triggers the opening of the AV valves.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the isovolumetric relaxation phase ends before the ventricles have fully relaxed and the AV valves have opened.
* **Option B:** This option is incorrect as it describes the isovolumetric contraction phase, which occurs earlier in the cardiac cycle.
* **Option C:** This option is incorrect because the isovolumetric relaxation phase is a distinct period that ends with the opening of the AV valves, not the closure of the semilunar valves.

**Clinical Pearl / High-Yield Fact**
The isovolumetric relaxation phase is a critical period for diastolic function, and any abnormalities in this phase can lead to diastolic dysfunction and heart failure. Understanding the cardiac cycle and its various phases is essential for diagnosing and managing cardiovascular diseases.

**Correct Answer: C. Opening of the AV valves.**