**Core Concept**
Mitral stenosis is a valvular heart disease characterized by the narrowing of the mitral valve opening, which obstructs blood flow from the left atrium to the left ventricle. The severity of mitral stenosis can be assessed based on clinical and echocardiographic findings, including valve area, pressure gradient across the valve, and symptoms.

**Why the Correct Answer is Right**
As the severity of mitral stenosis increases, several changes can be observed:

* The valve area decreases, leading to a higher pressure gradient across the valve.
* Left atrial size and pressure increase due to the obstruction of blood flow.
* Pulmonary venous congestion and pulmonary hypertension may develop.
* The classic symptoms of mitral stenosis, such as dyspnea, fatigue, and palpitations, become more pronounced.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because increased left atrial size is actually a feature of severe mitral stenosis, not a decrease.
**Option B:** Pulmonary hypertension is a common complication of severe mitral stenosis, so this option is also incorrect.
**Option D:** The absence of a characteristic "opening snap" on auscultation is a feature of severe mitral stenosis, not its absence.

**Clinical Pearl / High-Yield Fact**
In severe mitral stenosis, the left atrial pressure may become so high that it leads to left atrial thrombus formation, which can embolize to the lungs or systemic circulation.

**Correct Answer:** A. Increased left atrial size is a feature of severe mitral stenosis, not a decrease.