**Core Concept**
Mitral stenosis is a valvular heart disease characterized by the narrowing of the mitral valve opening, impeding blood flow from the left atrium to the left ventricle. The severity of mitral stenosis is crucial for determining the appropriate treatment and predicting patient outcomes.

**Why the Correct Answer is Right**
The severity of mitral stenosis is typically assessed using the mitral valve area (MVA), which is calculated using the continuity equation. The MVA is inversely related to the severity of stenosis, with smaller areas indicating more severe stenosis. The mitral valve area can be measured using transesophageal echocardiography (TEE) or cardiac catheterization. The American Society of Echocardiography recommends the use of the planimetry method for measuring MVA.

**Why Each Wrong Option is Incorrect**
* **Option A:** The left ventricular end-diastolic pressure (LVEDP) is not a direct measure of mitral stenosis severity, although it may be elevated in severe cases.
* **Option B:** The pulmonary capillary wedge pressure (PCWP) is a measure of left atrial pressure, but it does not directly assess the severity of mitral stenosis.
* **Option C:** The ejection fraction (EF) is a measure of left ventricular function, but it is not directly related to the severity of mitral stenosis.

**Clinical Pearl / High-Yield Fact**
The Gorlin formula is used to calculate the mitral valve area (MVA) based on the pressure gradient across the valve and the cardiac output. This formula is useful for estimating MVA when planimetry is not possible.

**Correct Answer:** D.