**Core Concept:** A Valsalva manoeuvre is a test performed by the patient to provoke a specific heart murmur, which is auscultated by the physician. It involves blowing into a sealed container (like a stopwatch or stethoscope) while maintaining a pressure differential between the mouth and the container. This test helps differentiate between aortic stenosis and mitral regurgitation.

**Why the Correct Answer is Right:** Aortic stenosis (AS) is a narrowing of the aortic valve, causing increased pressure in the left ventricle during systole. When the patient performs the Valsalva manoeuvre, the increased intra-thoracic pressure causes a decrease in the left atrial pressure, which leads to an increase in aortic pressure and causes the murmur to become louder.

**Why Each Wrong Option is Incorrect:**
A. Mitral regurgitation (MR) occurs when the mitral valve does not close properly, causing blood to flow back into the left atrium during diastole. The Valsalva manoeuvre causes an increase in left atrial pressure, which reduces the severity of MR murmur, not increase it.
B. Pulmonary hypertension (PH) is a condition characterized by increased pulmonary arterial pressure. The Valsalva manoeuvre increases pulmonary arterial pressure, which would decrease the severity of PH-related murmurs.
C. Aortic insufficiency (AI) is a condition characterized by a leaky aortic valve. The Valsalva manoeuvre increases aortic pressure, which would increase the severity of AI murmur.
D. Patent ductus arteriosus (PDA) is a congenital heart defect characterized by an open ductus arteriosus, allowing blood flow between the pulmonary artery and aorta. The Valsalva manoeuvre increases intra-thoracic pressure, which would increase the severity of a PDA murmur.

**Clinical Pearl:** Understanding the effect of the Valsalva manoeuvre on various murmurs is essential for proper heart examination interpretation and differential diagnosis in clinical practice.