A 35-year-old woman with a history of acute rheumatic fever presents with dyspnea on exertion. On examination, she has a loud S1, an opening snap, and a low-pitched diastolic murmur at the apex. Echocardiography confirms mitral stenosis. Which single finding best distinguishes mitral stenosis from mitral regurgitation?

## Distinguishing Mitral Stenosis from Mitral Regurgitation **Key Point:** The opening snap is pathognomonic for mitral stenosis and is absent in mitral regurgitation. It represents the abrupt halting of the mitral leaflets during early diastole due to the stenotic orifice. ### Comparison Table: MS vs MR | Feature | Mitral Stenosis | Mitral Regurgitation | | --- | --- | --- | | **Opening Snap** | Present (hallmark) | Absent | | **S1** | Loud (due to forceful closure) | Normal or soft | | **Diastolic Murmur** | Low-pitched, rumbling | Absent | | **Systolic Murmur** | Absent | Pansystolic (holosystolic) | | **Apex Beat** | Tapping, undisplaced | Hyperdynamic, displaced | | **Systolic Thrill** | Absent | May be present | | **S3 Gallop** | Absent (unless RV failure) | May be present (LV dilatation) | **High-Yield:** The opening snap occurs 40–100 ms after A2 and is best heard at the left lower sternal border with the diaphragm of the stethoscope. Its presence in a patient with a diastolic murmur at the apex is virtually diagnostic of mitral stenosis. **Clinical Pearl:** In mitral regurgitation, the left ventricle is volume-overloaded and dilated, leading to a hyperdynamic, displaced apex beat and often an S3 gallop. In mitral stenosis, the left ventricle is protected from volume overload by the stenotic mitral valve, so the apex remains undisplaced and tapping. **Mnemonic:** **OSMS** — Opening Snap = Mitral Stenosis (not regurgitation).