**Core Concept**
A murmur increasing with the Valsalva maneuver is indicative of a dynamic left ventricular outflow tract obstruction, typically seen in conditions such as hypertrophic obstructive cardiomyopathy (HOCM). This phenomenon occurs due to the increased afterload and decreased left ventricular volume during the Valsalva maneuver, which worsens the obstruction.

**Why the Correct Answer is Right**
The Valsalva maneuver increases intrathoracic pressure, reducing venous return to the heart and subsequently decreasing left ventricular volume. In patients with HOCM, the septal hypertrophy and abnormal papillary muscle insertion lead to a dynamic outflow tract obstruction, which worsens with decreased left ventricular volume. This results in an increase in the intensity of the murmur, typically a systolic ejection murmur, during the Valsalva maneuver.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a murmur increasing with the Valsalva maneuver is not characteristic of aortic stenosis, which typically worsens with increased left ventricular contraction and volume.
* **Option B:** This option is incorrect because a murmur increasing with the Valsalva maneuver is not characteristic of mitral regurgitation, which typically worsens with increased left ventricular volume and contraction.
* **Option C:** This option is incorrect because a murmur increasing with the Valsalva maneuver is not characteristic of patent ductus arteriosus, which typically presents with a continuous murmur throughout systole and diastole.

**Clinical Pearl / High-Yield Fact**
A classic clinical correlation for a murmur increasing with the Valsalva maneuver is a history of syncope or near-syncope with exertion, as the increased dynamic outflow tract obstruction can lead to decreased cardiac output and subsequent hypotension.

**Correct Answer:** C.