**Core Concept**
Mitral regurgitation (MR) is a condition where the mitral valve fails to close properly, allowing blood to leak backward from the left ventricle into the left atrium during systole. This can lead to an increased volume load on the left heart, potentially resulting in left-sided heart failure.

**Why the Correct Answer is Right**
In the case of congenital mitral regurgitation, the mitral valve leaflets may be abnormally shaped or positioned, or there may be other structural anomalies that cause the valve to malfunction. The physical findings may include a holosystolic murmur best heard at the apex with the patient in the left lateral decubitus position. This murmur is typically a high-pitched, blowing sound that radiates to the axilla. The murmur may be accompanied by a palpable thrill over the apex. The patient may also exhibit signs of left ventricular volume overload, such as an enlarged left ventricle on echocardiogram.

**Why Each Wrong Option is Incorrect**
* **Option A:** A pericardial rub would be more indicative of pericarditis, an inflammation of the pericardium, which would not be consistent with mitral regurgitation.
* **Option B:** A third heart sound (S3) may be present in conditions of volume overload, but it is not specific to mitral regurgitation and can be heard in other conditions such as heart failure or cardiomyopathy.
* **Option C:** A diastolic murmur would be more indicative of aortic stenosis or mitral stenosis, not mitral regurgitation.

**Clinical Pearl / High-Yield Fact**
In patients with mitral regurgitation, it is essential to listen for a holosystolic murmur at the apex, as this is a classic sign of the condition. The murmur may be accompanied by a palpable thrill and signs of left ventricular volume overload.

**Correct Answer:** D.