**Core Concept**
Mitral stenosis is a valvular heart disease characterized by the narrowing of the mitral valve orifice, leading to restricted blood flow from the left atrium to the left ventricle. The condition often results in increased pressure and volume within the left atrium, causing it to become enlarged.

**Why the Correct Answer is Right**
The double atrial shadow in mitral stenosis is primarily due to the enlargement of the left atrium, which can be visualized on chest X-rays or echocardiograms. The increased size of the left atrium can cause it to project beyond the right atrium, creating a double density or shadow on radiographic images. This is often referred to as "double atrial shadow" or "double density sign". The enlarged left atrium is a result of the increased pressure and volume within the chamber, which is a direct consequence of the mitral stenosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the double atrial shadow in mitral stenosis. While pericardial effusion can cause an enlarged cardiac silhouette, it is not directly related to the double atrial shadow seen in mitral stenosis.
* **Option B:** Right atrial enlargement is not a primary cause of the double atrial shadow in mitral stenosis. While the right atrium may also be enlarged in cases of heart failure or other conditions, it is not the primary contributor to the double density sign seen in mitral stenosis.
* **Option C:** Pulmonary hypertension is a potential complication of mitral stenosis, but it is not the cause of the double atrial shadow. The double density sign is primarily due to the enlargement of the left atrium, which is a direct result of the mitral stenosis.

**Clinical Pearl / High-Yield Fact**
The double atrial shadow is a classic radiographic finding in mitral stenosis, and its presence should raise suspicion for this condition. It is essential to consider the clinical context and other diagnostic findings when interpreting radiographic images to avoid missing this important diagnosis.

**Correct Answer:** C. Pulmonary hypertension.