**Core Concept**
Pneumoperitoneum refers to the presence of free air in the peritoneal cavity, often indicating a perforated viscus. The detection of free air relies on its ability to create a radiographic "air-fluid level" or "air under the diaphragm" when the patient is positioned appropriately. The left lateral decubitus view is most sensitive for detecting pneumoperitoneum due to gravity-dependent air accumulation.

**Why the Correct Answer is Right**
In a patient with suspected pneumoperitoneum, free air will settle in the dependent portion of the peritoneal cavity. When the patient is in a **left lateral decubitus position**, gravity causes air to pool in the left lower abdomen, where it can be visualized as a **biconvex air-fluid level** or **air under the diaphragm**. This view is superior to others because it maximizes the visibility of air in the peritoneal space. The **left** side is preferred because the liver is on the right, and the left side has more space for air to collect.

**Why Each Wrong Option is Incorrect**
Option B: Right lateral decubitus view is less effective because the liver occupies the right side, limiting space for air accumulation.
Option C: Supine view shows air only if it's in the midline or under the diaphragm, but it fails to demonstrate gravity-dependent pooling.
Option D: Prone view is not standard for pneumoperitoneum and does not allow optimal visualization of air under the diaphragm.

**Clinical Pearl / High-Yield Fact**
Always perform a **left lateral decubitus view** in a patient with suspected pneumoperitoneum — it is the **gold standard** for detecting free air due to gravity-dependent air pooling. A **biconvex air-fluid level** or **air under the diaphragm** is diagnostic.

✓ Correct Answer: A. Left lateral decubitus view