**Core Concept**
Mitral stenosis is a valvular heart disease characterized by the narrowing of the mitral valve opening, which restricts blood flow from the left atrium to the left ventricle. This condition can lead to various radiological findings, particularly in chest X-rays and echocardiograms.

**Why the Correct Answer is Right**
The correct answer includes all the given options because mitral stenosis can manifest through several distinct radiological features. The "double density sign" or "double contour" of the right heart border (Option A) is due to the enlarged left atrium, which appears as a second cardiac contour. The straightening of the left heart border (Option B) is also a result of the enlarged left atrium. The splaying of the carinal angle (Option C) occurs due to the upward displacement and enlargement of the left atrium, which pushes the carina (the tracheal bifurcation) apart.

**Why Each Wrong Option is Incorrect**
* **Option A:** The "double density sign" is not unique to mitral stenosis; it can be seen in other conditions, such as left atrial enlargement due to other causes. However, in the context of mitral stenosis, it is a significant finding.
* **Option B:** While straightening of the left heart border can occur in mitral stenosis, it is not a specific or exclusive feature of this condition. Other causes of left atrial enlargement can also lead to this finding.
* **Option C:** Splaying of the carinal angle is indeed a feature of mitral stenosis, but it is not the most distinctive or sensitive indicator of this condition.

**Clinical Pearl / High-Yield Fact**
In cases of suspected mitral stenosis, it is essential to look for multiple radiological features, as a single finding may not be diagnostic. The presence of a "double density sign" or "double contour" of the right heart border, straightening of the left heart border, and splaying of the carinal angle together strongly suggest mitral stenosis.

✓ Correct Answer: D. All