## **Core Concept**
The right side of the mediastinal shadow on a chest radiograph is primarily formed by the superior and inferior vena cava, the right atrium, and the inferior border of the right brachiocephalic vein (or the right innominate vein) and sometimes the ascending aorta. Understanding the anatomical structures that contribute to the mediastinal contours is crucial for interpreting chest radiographs.

## **Why the Correct Answer is Right**
The correct answer, , represents the structure that does not contribute to the right mediastinal border. Typically, the right mediastinal border is formed by the superior vena cava, right atrium, and inferior vena cava. The azygos vein, which is a structure that can contribute to the right mediastinal border, especially its upper part, is not listed, making the option related to it potentially correct. The right main pulmonary artery does not form the right border; it forms part of the cardiac silhouette but is not on the right mediastinal border.

## **Why Each Wrong Option is Incorrect**
* **Option A:** The superior vena cava does form part of the right mediastinal border.
* **Option B:** The inferior vena cava also contributes to the right mediastinal border, particularly its lower part.
* **Option D:** The right atrium is a major component of the right heart border.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the right mediastinal border is primarily made up of the superior vena cava, right atrium, and inferior vena cava. When evaluating a chest X-ray, abnormalities in these structures, such as an enlarged cardiac silhouette or abnormal vascular markings, can indicate various cardiovascular or pulmonary conditions.

## **Correct Answer: .**