What is the structure ‘B’ marked in lateral chest X-ray below?
**Core Concept**
The structure marked 'B' in the lateral chest X-ray is likely a part of the mediastinum, which is the central portion of the thoracic cavity containing various vital organs, including the heart, trachea, esophagus, and major blood vessels. The mediastinum is divided into anterior, middle, and posterior compartments, each containing distinct structures.
**Why the Correct Answer is Right**
The correct answer is related to the aortic arch, which is a major artery that arises from the ascending aorta and curves around the left main bronchus to form the descending aorta. The aortic arch is a key landmark in the mediastinum and can be visualized on a lateral chest X-ray. The aortic arch is responsible for supplying oxygenated blood to the head, neck, and upper limbs.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the aortic knob is a structure that can be seen on a chest X-ray, but it is not the correct answer in this context. The aortic knob is the dilated portion of the aortic arch where it gives off the brachiocephalic artery.
* **Option B:** This is incorrect because the left main bronchus is not the correct structure marked 'B'. The left main bronchus is a part of the respiratory system and is not typically visualized on a lateral chest X-ray.
* **Option C:** This is incorrect because the esophagus is not the correct structure marked 'B'. The esophagus is a muscular tube that carries food from the pharynx to the stomach, and while it can be visualized on a barium swallow study, it is not typically seen on a lateral chest X-ray.
**Clinical Pearl / High-Yield Fact**
In a lateral chest X-ray, the aortic arch can be identified by its characteristic "3" shape, with the aortic knob forming the top of the "3" and the descending aorta forming the bottom.
**Correct Answer: C. Esophagus**