## **Core Concept**
The intensity of a systolic murmur can be influenced by various factors, including changes in ventricular volume, contractility, and heart rate. Conditions that affect these parameters can alter the intensity of systolic murmurs.

## **Why the Correct Answer is Right**
The correct answer, **C. Valsalva maneuver**, involves a forced expiration against a closed glottis, which decreases venous return to the heart, subsequently reducing ventricular volume. This reduction in ventricular volume increases the dynamic obstruction in conditions like hypertrophic cardiomyopathy (HCM), thereby increasing the intensity of the systolic murmur. This is particularly notable in HCM, where the murmur intensity can increase with maneuvers that decrease ventricular volume.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Increased preload* (e.g., with fluid infusion) typically decreases the intensity of systolic murmurs associated with dynamic obstruction (like in HCM) because it increases the ventricular volume, reducing the obstruction.
- **Option B:** *Phenylephrine* (an alpha-1 agonist) can increase afterload and potentially decrease the intensity of some systolic murmurs by reducing the contractility and velocity of blood flow through the obstruction.
- **Option D:** *Atrial fibrillation* often results in an irregular heart rhythm and varying ventricular filling, which can lead to beat-to-beat variability in murmur intensity but generally does not consistently increase the intensity of systolic murmurs.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the intensity of a systolic murmur in **hypertrophic cardiomyopathy (HCM)** can be increased by maneuvers that decrease ventricular volume (e.g., Valsalva maneuver, standing) and decreased by maneuvers that increase ventricular volume (e.g., squatting, leg raising). This characteristic response can help differentiate HCM from other causes of systolic murmurs.

## **Correct Answer:** C. Valsalva maneuver.