**Core Concept**
The detection of pneumoperitoneum, which is the presence of free air within the abdominal cavity, is crucial for diagnosing conditions such as perforated ulcers. This can be achieved through various imaging modalities, each with its own advantages and limitations.

**Why the Correct Answer is Right**
The lateral decubitus position is the most sensitive and specific method for detecting pneumoperitoneum. When a patient is positioned in a lateral decubitus position, the free air rises and accumulates under the diaphragm, making it more visible on an upright chest or abdominal radiograph. This is because the air rises to the highest point in the abdominal cavity, which in this position is under the diaphragm, allowing for easier detection. The sensitivity and specificity of this method make it the best choice for detecting pneumoperitoneum.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not the best test for detecting pneumoperitoneum, as it may not always show the free air, especially if the patient is positioned incorrectly or if the air is not in the optimal position for detection.

**Option B:** X-ray abdomen in supine position is not the best test for detecting pneumoperitoneum, as the free air may not be visible in this position, as it tends to accumulate under the diaphragm.

**Option C:** This option is repeated, but it's the same as option B, and the reasoning is the same.

**Clinical Pearl / High-Yield Fact**
The lateral decubitus position is the best method for detecting pneumoperitoneum because it takes advantage of the natural tendency of free air to rise and accumulate under the diaphragm, making it easier to visualize on an upright radiograph.

**Correct Answer:**
✓ Correct Answer: D. X-ray abdomen in lateral decubitus