## Core Concept
The posteroanterior (PA) view of a chest X-ray is a crucial diagnostic tool for evaluating the cardiac silhouette. The borders of the heart in a PA chest X-ray are typically formed by the right atrium, right ventricle, left atrium, and left ventricle, with specific structures contributing to the right and left heart borders.

## Why the Correct Answer is Right
The right border of the heart is typically formed by the **superior vena cava (SVC)**, **right atrium (RA)**, and **inferior vena cava (IVC)**. The left border is primarily formed by the **left ventricle (LV)** and the **left subclavian artery** (or aortic arch). The **aortic arch** and **main pulmonary artery** contribute to the left upper and middle cardiac borders, respectively. The **right ventricle** does not directly form the border of the heart silhouette in a standard PA chest X-ray because it is anterior and does not contribute to the cardiac silhouette's outline.

## Why Each Wrong Option is Incorrect
* **Option A:** The superior vena cava does form part of the right heart border.
* **Option B:** The inferior vena cava also forms part of the right heart border.
* **Option C:** The left ventricle forms the majority of the left heart border.

## Clinical Pearl / High-Yield Fact
A key point to remember is that in a PA chest X-ray, the cardiac silhouette's borders are primarily made up of the chambers and great vessels. The right ventricle, being more anterior, does not contribute to the silhouette's border, making it a common pitfall.

## Correct Answer: D. Right ventricle.