A 58-year-old male with a history of chronic NSAID use for osteoarthritis presents to the emergency department with sudden onset severe epigastric pain and board-like abdominal rigidity. An erect chest X-ray is obtained. The structure marked **A** in the diagram—a lucent crescent of air under the right hemidiaphragm—is identified. Which of the following is the most likely diagnosis and the primary etiology of this radiological finding?

Explanation

## Why "Perforated duodenal ulcer due to NSAID-induced mucosal erosion causing pneumoperitoneum" is right The clinical presentation—sudden severe epigastric pain, board-like rigidity, and chronic NSAID use—combined with the radiological finding of a lucent crescent of air under the right hemidiaphragm (structure **A**) is pathognomonic for perforated peptic ulcer. Perforated peptic ulcer is the #1 spontaneous cause of pneumoperitoneum, with duodenal perforation more common than gastric. NSAIDs are a major risk factor. The erect chest X-ray is the simplest diagnostic test to detect free intraperitoneal gas, which appears as a lucent crescent beneath the hemidiaphragm (right side more reliable than left, which may be obscured by the gastric bubble). This finding indicates acute peritonitis requiring urgent surgical intervention (Greenfield Surgery 7e Ch 24). ## Why each distractor is wrong - **Perforated sigmoid diverticulitis with fecal peritonitis and secondary air dissection**: While perforated diverticulitis is a cause of pneumoperitoneum, it typically occurs in elderly patients and is the #2 cause, not #1. The clinical presentation of sudden epigastric pain with NSAID history strongly points to peptic ulcer disease, not diverticulitis (which presents with left lower quadrant pain and fever over days). - **Ischemic bowel necrosis with transmural perforation and gas tracking**: Ischemic bowel perforation is a less common cause of pneumoperitoneum and typically occurs in patients with vascular disease, atrial fibrillation, or shock. The acute epigastric presentation with NSAID history is not consistent with mesenteric ischemia. - **Iatrogenic perforation following recent colonoscopy with polypectomy**: Although post-endoscopic perforation is a recognized cause of pneumoperitoneum, the clinical history provided (chronic NSAID use, sudden epigastric pain) strongly suggests spontaneous peptic ulcer perforation rather than an iatrogenic event. There is no mention of recent endoscopy. **High-Yield:** Sudden severe epigastric pain + board-like rigidity + lucent crescent under right hemidiaphragm on erect CXR = perforated peptic ulcer (duodenal > gastric) until proven otherwise; NSAIDs and H. pylori are major risk factors; this is a surgical emergency. [cite: Greenfield Surgery 7e Ch 24]