## **Core Concept**
The question pertains to the cardiac cycle, specifically focusing on the events occurring during the isometric relaxation phase. The cardiac cycle consists of several phases, including isovolumic (isometric) contraction and relaxation. Isometric relaxation is the phase 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 event that occurs at the end of the isometric relaxation phase. During isometric relaxation, the ventricular pressure decreases rapidly until it falls below the atrial pressure. The key event marking the end of isometric relaxation is the opening of the mitral valve, allowing blood to flow from the atrium into the ventricle. This is a critical moment in the cardiac cycle as it signifies the transition from diastole's early phase to the rapid filling phase of diastole.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option is incorrect because it does not accurately represent the event at the end of isometric relaxation.
- **Option B:** - This is incorrect because it corresponds to another phase or event in the cardiac cycle not related to the end of isometric relaxation.
- **Option C:** - Incorrect as it does not mark the end of isometric relaxation.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the cardiac cycle's phases are intricately linked with heart sounds. The end of isometric relaxation and the beginning of the rapid filling phase can be associated with the third heart sound (S3), which occurs when blood rapidly fills the ventricle.

## **Correct Answer Line**
**Correct Answer: D. **