**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. This condition leads to an increase in left atrial pressure and, consequently, an increase in the size of the left atrium.

**Why the Correct Answer is Right**
In mitral stenosis, the left atrium is subjected to increased pressure due to the obstruction of blood flow through the mitral valve. As a result, the left atrium undergoes hypertrophy (an increase in muscle mass) and dilation (an increase in size). This is because the left atrium is responsible for pumping blood into the left ventricle, and any obstruction to this process will lead to an increase in left atrial pressure and, subsequently, an increase in its size. The increased pressure in the left atrium also leads to the development of left atrial hypertrophy and dilation, which can eventually lead to left atrial enlargement.

**Why Each Wrong Option is Incorrect**
**Option B:** The right atrium is not directly affected by mitral stenosis, as it is the left atrium that experiences increased pressure due to the obstruction of blood flow through the mitral valve.

**Option C:** The left ventricle is not enlarged in the early stages of mitral stenosis, as the obstruction to blood flow is located in the mitral valve, not in the left ventricle itself.

**Option D:** The right ventricle is not affected by mitral stenosis, as the condition primarily involves the left side of the heart, specifically the mitral valve and the left atrium.

**Clinical Pearl / High-Yield Fact**
In patients with mitral stenosis, the left atrium is often enlarged, and this can be detected on a chest X-ray or echocardiogram. The left atrial enlargement is a direct consequence of the increased pressure in the left atrium due to the obstruction of blood flow through the mitral valve.

**✓ Correct Answer: A. Left atrium**