Sound is that of S1–A2–OS with a relatively long A2–OS interval. The opening snap (OS), most commonly caused by mitral stenosis, is thought to be caused by abrupt downward bulging (snapping) of the anterior leaflet as left ventricular pressure drops below left atrial pressure during diastole. A2–OS can be distinguished from a split S2 by dynamic maneuvers (OS intensity increases with inspiration, A2–OS interval widens with standing), a triple S2 (ie, A2–P2–OS), and a louder volume at the apex.