What is the most likely diagnosis for a thin woman presenting with a sharp click between heart sounds and a sternal depression?

The presence of a sharp click between heart sounds, particularly in a thin woman, is highly indicative of mitral valve prolapse. This condition occurs when the mitral valve's leaflets become enlarged or elongated, causing them to slip back into the left atrium during systole. The "click" sound occurs due to the sudden tension in the chordae tendineae as the valve prolapses.

In addition to the click, the description of a sternal depression may align with the classic physical characteristic of individuals who are more slender, which can sometimes be seen in those with mitral valve prolapse. This demographic correlation, along with the auscultatory finding, supports mitral valve prolapse as the most likely diagnosis.

While the other conditions could also present with murmurs or other heart sounds, they lack the characteristic sharp click associated with mitral valve prolapse. Aortic stenosis is typically associated with a systolic ejection murmur and does not present with a click. Tricuspid regurgitation and pulmonary valve stenosis are associated with different types of heart sounds and do not commonly produce a sharp click during the cardiac cycle. Therefore, the specific findings in this case point clearly towards mitral valve prolapse as the most appropriate diagnosis