What is the most likely diagnosis for an asymptomatic teenager presenting with a very loud pansystolic murmur at the lower left sternal edge?

In the context of an asymptomatic teenager exhibiting a very loud pansystolic murmur at the lower left sternal edge, the most likely diagnosis is a small ventricular septal defect (VSD).

A small VSD often produces a characteristic loud pansystolic (also known as holosystolic) murmur that is heard best at the lower left sternal border. This is due to the high pressure in the left ventricle during systole, leading to a significant left-to-right shunt through the defect, which generates turbulence and produces the murmur. The absence of symptoms in this case aligns with the typical presentation of a small VSD, where many individuals may remain asymptomatic, especially if the defect is small and the volume overload on the heart is minimal.

This presentation is distinct from other conditions, such as mitral regurgitation, which typically presents with a different murmur characteristic, usually best heard at the apex and may involve symptoms like fatigue or dyspnea. Aortic stenosis would present with a systolic ejection murmur rather than a pansystolic one, often associated with symptoms of exertional dyspnea or syncope in older individuals. Tricuspid regurgitation, producing a holos