**Core Concept**
The murmur associated with hypertrophic cardiomyopathy (HCM) is a result of dynamic left ventricular outflow tract (LVOT) obstruction. In HCM, the thickened left ventricular wall causes obstruction to blood flow, leading to a systolic ejection murmur.

**Why the Correct Answer is Right**
The murmur of HCM is typically a harsh, late-peaking systolic ejection murmur heard best at the left sternal border. This murmur is caused by the obstruction of the LVOT, which is exacerbated by factors such as increased contractility, decreased preload, and increased afterload. The murmur is decreased in conditions that reduce LVOT obstruction, such as decreased contractility or increased preload.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because increased contractility exacerbates LVOT obstruction and increases the murmur.
**Option B:** Incorrect because increased afterload also increases LVOT obstruction and the murmur.
**Option C:** Incorrect because this is not a condition that typically affects LVOT obstruction.

**Clinical Pearl / High-Yield Fact**
Decreased contractility, as seen in conditions such as beta-blocker therapy or cardiogenic shock, can decrease the murmur of HCM by reducing LVOT obstruction.

**Correct Answer:** D. Decreased contractility.