**Core Concept**
The right border of the mediastinum on a chest X-ray is formed by several key structures, including the **superior vena cava**, **right atrium**, and **inferior vena cava**. Understanding the anatomical composition of the mediastinum is crucial for interpreting chest radiographs. The mediastinum contains vital organs and vessels.

**Why the Correct Answer is Right**
The correct answer choice should be the one that does not contribute to the formation of the right mediastinal border. Normally, the right border is formed by the **superior vena cava**, **right atrium**, and **inferior vena cava**. Any structure not typically visible or not contributing to this border would be the correct answer.

**Why Each Wrong Option is Incorrect**
**Option A:** Typically, this would be a structure that does contribute to the right mediastinal border, such as the **superior vena cava**.
**Option B:** This could be another structure like the **right atrium** that forms part of the border.
**Option C:** Similarly, this might be a structure that contributes to the border, like the **inferior vena cava**.

**Clinical Pearl / High-Yield Fact**
Remembering that the **silhouette sign** can help identify mediastinal masses or enlargements is key. A structure that does not normally form part of the right border could, if enlarged, create an abnormal silhouette on a chest X-ray.

**Correct Answer:** D.