**Core Concept**
The mediastinum is the central part of the chest cavity, containing the heart, trachea, esophagus, and other vital structures. On a PA (posteroanterior) chest radiograph, the mediastinum is divided into three main regions: the right, left, and middle mediastinum. Each region contains specific anatomical structures that help in identifying them on a chest X-ray.

**Why the Correct Answer is Right**
The left mediastinal border is formed by several structures, including the aortic arch, the main pulmonary artery, the left ventricle, and the left atrium. These structures are located in the left mediastinum and are visible on a PA chest radiograph.

**Why Each Wrong Option is Incorrect**
**Option A:** The aortic arch is a part of the left mediastinal border. It is the uppermost part of the aorta that curves over the left main bronchus and is visible on a PA chest radiograph.
**Option B:** The main pulmonary artery is also a part of the left mediastinal border. It arises from the right ventricle and descends to the left, forming the left pulmonary artery, which is visible on a PA chest radiograph.
**Option C:** The left ventricle is another structure that contributes to the left mediastinal border. It is the larger of the two ventricles and is visible on a PA chest radiograph.

**Clinical Pearl / High-Yield Fact**
In a normal chest radiograph, the left mediastinal border is usually concave, whereas the right mediastinal border is convex. This is due to the location of the aortic arch and the main pulmonary artery on the left side.

**Correct Answer:** D.