Okay, let's tackle this question about the isovolumic relaxation phase of the cardiac cycle. The question is asking when this phase ends. First, I need to recall the sequence of events in the cardiac cycle.

The isovolumic relaxation phase occurs after the ventricles have finished ejecting blood during systole. Once the ventricular systole is over, the ventricles start to relax. During this relaxation, the pressure inside the ventricles drops. The key here is that the volume of the ventricles remains constant during this phase because both the aortic/pulmonary valves and the mitral/tricuspid valves are closed. So, the phase is called "isovolumic" because the volume doesn't change.

Now, when does this phase end? The isovolumic relaxation ends when the ventricular pressure drops below the atrial pressure. At that point, the mitral or tricuspid valve (AV valves) opens, allowing blood to flow from the atria into the ventricles. This marks the beginning of the ventricular filling phase. So the end of isovolumic relaxation is when the AV valves open.

Looking at the options, the correct answer should be the opening of the AV valves. The other options might include events like the aortic valve closing (which marks the start of isovolumic relaxation), the opening of the semilunar valves (which would be during ejection), or maybe the start of atrial systole. Each of these is a different event in the cycle.

I need to make sure I'm not confusing the start and end of the phase. The isovolumic relaxation starts when the semilunar valves close (end of ejection) and ends when the AV valves open (start of filling). So the ending is indeed the opening of the AV valves.

**Core Concept**
The isovolumic relaxation phase of the cardiac cycle occurs when ventricular pressure falls below aortic and pulmonary pressures but remains above atrial pressure, causing all valves to remain closed. It ends when ventricular pressure drops below atrial pressure, allowing atrioventricular (AV) valve opening. This phase is critical for understanding ventricular filling dynamics.

**Why the Correct Answer is Right**
The isovolumic relaxation phase ends when the **mitral (left) or tricuspid (right) AV valves open**, permitting blood flow from the atria into the ventricles. This occurs as ventricular pressure falls below atrial pressure (~7 mmHg in the left atrium), overcoming AV valve closure. The phase is "isovolumic" because ventricular volume remains constant until the AV valves open, after which rapid filling begins. This transition marks the start of diastole.

**Why Each Wrong Option is Incorrect**
**Option A:** *Aortic valve closure* marks the **start** of isovolumic relaxation, not the end.
**Option B:** *Opening of the semilunar valves* occurs during **ventricular ejection phase**, not relaxation.
**Option C:** *Atrial systole* occurs during **ventricular filling** and is unrelated to isovolumic relaxation termination.

**Clinical Pearl / High-Yield Fact**
Remember the cardiac cycle sequence: **Isovolumic relaxation ends with AV valve opening**, followed by rapid ventricular filling. Confusing this with

✓ Correct Answer: B. Opening of A.V valve