**Core Concept**
Mitral stenosis is a valvular heart disease characterized by narrowing of the mitral valve orifice, leading to impaired blood flow from the left atrium to the left ventricle. Severe mitral stenosis results in increased pressure and volume overload in the left atrium, which can lead to various complications and clinical manifestations.

**Why the Correct Answer is Right**
The opening snap (OS) is a characteristic auscultatory finding in mitral stenosis, produced by the sudden closure of the mitral valve after the pressure gradient between the left atrium and ventricle has been established. In severe mitral stenosis, the OS is typically _earlier_ than the second heart sound (S2), not delayed. This is because the increased pressure in the left atrium causes the mitral valve to open earlier, resulting in a closer approximation of the OS to S2.

**Why Each Wrong Option is Incorrect**
**Option A:** A pulsatile liver is a sign of right-sided heart failure, which can occur in severe mitral stenosis due to increased pressure and volume overload in the left atrium and subsequently in the pulmonary veins and capillaries, leading to congestion and hepatomegaly.
**Option B:** Atrial fibrillation is a common complication of mitral stenosis, particularly in the advanced stages, due to increased pressure and volume overload in the left atrium, leading to electrical instability and arrhythmias.
**Option D:** An increase in the length of the murmur is a characteristic feature of severe mitral stenosis, as the increased pressure gradient between the left atrium and ventricle prolongs the duration of the murmur.

**Clinical Pearl / High-Yield Fact**
In mitral stenosis, the opening snap is typically associated with a low-pitched, high-frequency sound, best heard at the apex with the patient in the left lateral decubitus position.

**✓ Correct Answer:** C. Opening snap delayed from S2