## Core Concept
The cardiac cycle consists of several phases, including isovolumic contraction, ventricular ejection, isovolumic relaxation, and ventricular filling. The opening of the aortic valve is a critical event that occurs during the ventricular ejection phase.

## Why the Correct Answer is Right
The aortic valve opens when the pressure in the left ventricle exceeds the pressure in the aorta, allowing blood to be ejected into the systemic circulation. This event marks the beginning of the ventricular ejection phase, which is divided into two parts: rapid ejection and reduced ejection. The rapid increase in ventricular pressure during isovolumic contraction eventually surpasses aortic pressure, leading to the opening of the aortic valve.

## Why Each Wrong Option is Incorrect
* **Option A:** Isovolumic contraction is the phase where the ventricles contract and increase the pressure inside the ventricles without ejecting blood because the pressure inside the ventricles has not yet exceeded the pressure in the great arteries. The aortic valve remains closed during this phase.
* **Option B:** Isovolumic relaxation is the phase following ventricular ejection where the ventricles relax, and the pressure inside the ventricles decreases rapidly. The aortic valve closes at the end of ventricular ejection and remains closed during isovolumic relaxation.
* **Option D:** Ventricular filling is the phase where the ventricles relax and fill with blood. This phase includes the rapid filling, diastasis, and atrial contraction. The aortic valve is closed during ventricular filling.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the opening and closing of heart valves are directly related to the pressure differences across them. The sequence of valve openings and closures defines the different phases of the cardiac cycle. Specifically, the aortic valve opens with the start of ventricular ejection and closes when the ventricular pressure falls below the aortic pressure.

## Correct Answer: C. Ventricular ejection.