Right side of mediastinal shadow is NOT formed by: September 2012
**Core Concept**
The mediastinal shadow on a chest X-ray is divided into three main compartments: anterior, middle, and posterior. The right side of the mediastinal shadow is formed by the combination of several structures, including the right atrium, superior vena cava, and right main bronchus.
**Why the Correct Answer is Right**
The right main bronchus is a key structure that contributes to the formation of the right side of the mediastinal shadow. It arises from the right lung and extends upwards towards the trachea, forming a portion of the right border of the mediastinum. The right main bronchus is wider and more vertical than the left main bronchus, which makes it a prominent landmark on chest X-rays.
**Why Each Wrong Option is Incorrect**
* **Option A:** The right atrium is a major contributor to the formation of the right side of the mediastinal shadow. It is a chamber of the heart that receives deoxygenated blood from the body and pumps it to the lungs through the pulmonary veins.
* **Option B:** The superior vena cava is another key structure that contributes to the formation of the right side of the mediastinal shadow. It is a large vein that carries deoxygenated blood from the upper body to the right atrium.
* **Option D:** The right pulmonary artery is a branch of the pulmonary artery that arises from the right ventricle and supplies oxygenated blood to the right lung. While it is a part of the mediastinal shadow, it is not a primary contributor to the formation of the right side of the mediastinal shadow.
**Clinical Pearl / High-Yield Fact**
The right main bronchus is wider and more vertical than the left main bronchus, which makes it a useful landmark for identifying the right border of the mediastinum on chest X-rays.
**Correct Answer:** A. The right atrium is a major contributor to the formation of the right side of the mediastinal shadow.