A 50 year old man with acute abdominal pain

The correct answer is B. The most likely cause of the pneumoperitoneum is a perforated peptic ulcer. This patient has a history of indigestion and takes regular anti inflammatory drugs. The supine x-ray of the abdomen shows normal gas in the bowel and free air outside of the bowel. This is known as Rigler's sign. Rigler's sign is usually seen when there is more than one thousandmls of free gas and fluid in the peritoneum. There is also a pocket of air in the right iliac fossa between the loops of bowel. This is why the following answers are incorrect. A. There is no history of diverticular disease in this patient. This patient has a history of indigestion and takes regular anti inflammatory drugs. The most likely cause is a perforated peptic ulcer. C. Free intraperitoneal gas is the striking feature on this X-ray, which is not characteristic of spontaneous bacterial peritonitis.