True about third heart sound is -a) Absent in Chr. constrictive pericariditisb) Absent in aortic aneurysmc) Absent in MSd) Normal physiologically in Athletes

**Core Concept**
The third heart sound (S3) is a low-frequency sound heard during early diastole, typically associated with rapid filling of the ventricles. It is often a sign of increased ventricular volume or compliance, which can be seen in various cardiac conditions.

**Why the Correct Answer is Right**
The third heart sound is generated by the rapid filling of the ventricle, particularly during the early diastolic phase. In conditions like mitral stenosis (MS), the increased pressure gradient across the mitral valve leads to a more forceful closure of the valve, resulting in an accentuated S3 sound. This is because the ventricle is trying to fill more quickly to compensate for the restricted valve orifice.

**Why Each Wrong Option is Incorrect**
**Option A:** Chr. constrictive pericarditis - In constrictive pericarditis, the pericardium becomes fibrotic and non-compliant, restricting ventricular filling. However, this condition is more likely to produce a pericardial knock rather than an absent S3 sound.

**Option B:** Aortic aneurysm - An aortic aneurysm is a dilation of the aorta, which can lead to a variety of symptoms, but it is not directly related to the presence or absence of an S3 sound.

**Option C:** Mitral stenosis - As mentioned earlier, MS is actually associated with an accentuated S3 sound due to the increased pressure gradient across the mitral valve.

**Option D:** Normal physiologically in Athletes - In athletes, the S3 sound is often present due to the increased ventricular volume and compliance associated with regular exercise.

**Clinical Pearl / High-Yield Fact**
The S3 sound is a sensitive indicator of increased ventricular volume or compliance, making it a useful clinical tool in diagnosing conditions like MS.

**Correct Answer: C. Mitral stenosis**