## **Core Concept**
The cardiac cycle consists of several phases, including isovolumic (isometric) contraction and relaxation. The isometric relaxation phase is the period when the ventricles start to relax, but the volume remains constant because both the mitral and aortic valves are closed.

## **Why the Correct Answer is Right**
The correct answer, , corresponds to the third heart sound (S3). The third heart sound occurs at the end of the isometric relaxation phase, just before the mitral valve opens and rapid filling of the ventricle begins. This sound is associated with the rapid filling of the ventricles and is a normal finding in children and young adults but can also be a sign of heart failure in adults.

## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option does not correspond to a commonly recognized cardiac event related to the phases of the cardiac cycle in the context provided.
* **Option B:** - This option is incorrect because the second heart sound (S2) occurs at the end of systole, marking the closure of the aortic and pulmonary valves, which is not at the end of the isometric relaxation phase.
* **Option D:** - This option does not accurately represent a cardiac event specifically tied to the end of the isometric relaxation phase.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the third heart sound (S3) is often associated with conditions that lead to increased fluid volume or rapid heart rate, such as heart failure. Its presence can indicate increased ventricular volume and is a marker of ventricular dysfunction.

## **Correct Answer:** . Third heart sound (S3).