## **Core Concept**
The presence of gas under the diaphragm, also known as pneumoperitoneum, is a radiographic finding that often indicates a perforated abdominal viscus. This condition allows free air to escape into the abdominal cavity, which can then rise to accumulate under the diaphragm. The most common causes are related to gastrointestinal perforation.

## **Why the Correct Answer is Right**
The correct answer, **C. Perforated peptic ulcer**, is the most common cause of pneumoperitoneum. This occurs when a peptic ulcer erodes through the stomach or duodenal wall, releasing gastric contents and air into the peritoneal cavity. The perforation of a peptic ulcer is a medical emergency requiring immediate surgical intervention. The mechanism involves the ulceration process weakening the wall of the stomach or duodenum, leading to perforation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Spontaneous perforation of a tumor** can indeed cause pneumoperitoneum, it is much less common compared to a perforated peptic ulcer. Tumors can erode through the bowel wall, but this is not the most frequent cause.
- **Option B:** **Ruptured diverticulitis** can also lead to pneumoperitoneum. However, diverticulitis is more commonly associated with localized inflammation and not as frequently with free perforation into the peritoneum compared to peptic ulcer disease.
- **Option D:** **Intestinal obstruction** can lead to ischemia and perforation of the bowel, causing pneumoperitoneum. However, this sequence of events is less direct and less common as an immediate cause compared to a perforated peptic ulcer.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that not all cases of pneumoperitoneum require immediate surgery; however, a perforated peptic ulcer is a classic surgical emergency. Clinicians must quickly assess the stability of the patient and the likelihood of the cause being a perforated viscus. A classic exam trap is to assume all pneumoperitoneum is due to a perforated peptic ulcer without considering other causes.

## **Correct Answer:** C. Perforated peptic ulcer