**Core Concept**
Hypertrophic obstructive cardiomyopathy (HOCM) is characterized by thickening of the heart muscle, leading to obstruction of blood flow out of the heart. The murmur associated with HOCM is a result of the dynamic outflow tract obstruction.

**Why the Correct Answer is Right**
The murmur of HOCM is typically a systolic ejection murmur that is best heard at the left sternal border. It is caused by the narrowing of the left ventricular outflow tract (LVOT) due to the thickened heart muscle. The murmur is decreased in situations where there is a decrease in the gradient across the LVOT, such as in **Option C: squatting**. When a person squats, the increase in intrathoracic pressure and the decrease in systemic vascular resistance decrease the gradient across the LVOT, resulting in a decrease in the murmur.

**Why Each Wrong Option is Incorrect**
* **Option A:** Standing or lying down would actually increase the murmur of HOCM, as these positions increase the gradient across the LVOT.
* **Option B:** Exercise or stress would also increase the murmur of HOCM, as it increases the contractility of the heart muscle and the gradient across the LVOT.

**Clinical Pearl / High-Yield Fact**
The murmur of HOCM is often described as a harsh, crescendo-decrescendo systolic murmur that is best heard at the left sternal border. It is a classic example of a dynamic outflow tract obstruction, and its characteristics can be modified by maneuvers that affect the gradient across the LVOT.

**Correct Answer: C. Squatting decreases the murmur of HOCM by decreasing the gradient across the left ventricular outflow tract.**