An erect abdominal X-ray is performed as part of the acute abdomen series in a 62-year-old man with acute onset epigastric pain and vomiting. The structure marked **B** (right hemi-diaphragm) shows no free gas beneath it. Which of the following clinical scenarios is MOST consistent with this finding?

Explanation

## Why "Mechanical small bowel obstruction from adhesions in a previously operated abdomen" is right The absence of free gas beneath the right hemi-diaphragm (marked **B**) rules out pneumoperitoneum, which would appear as a "crescent of air" or lucency under the diaphragm on an erect chest X-ray. This negative finding is critical because it excludes perforated viscus. In the clinical context of acute epigastric pain and vomiting, the next most likely diagnosis is mechanical small bowel obstruction. On erect films, this presents with multiple step-ladder air-fluid levels at different heights, a hallmark of small bowel obstruction. Adhesions are the most common cause in previously operated patients (Sutton Radiology 7e, Ch 27; Bailey & Love 28e, Ch 71). ## Why each distractor is wrong - **Perforated peptic ulcer requiring urgent surgical intervention**: Peptic ulcer perforation is the most common cause of pneumoperitoneum and would produce free gas beneath the diaphragm (visible as a crescent of air under **B**). The absence of this finding excludes perforation. - **Acute pancreatitis with sentinel loop formation**: While pancreatitis can present with epigastric pain and may show a sentinel loop (focal ileus) on X-ray, it does not typically present with the acute vomiting pattern described here and would not be the primary diagnosis in this scenario. The erect film's main utility is to exclude perforation and obstruction. - **Acute appendicitis with loss of the right properitoneal fat stripe**: Appendicitis is a right lower quadrant process and would not typically present with epigastric pain and vomiting as the primary complaint. Loss of the properitoneal fat stripe is a secondary sign of inflammation, not a primary diagnosis. **High-Yield:** On erect abdominal X-ray, the absence of free gas beneath the diaphragm excludes pneumoperitoneum (perforation); multiple step-ladder air-fluid levels at different heights indicate mechanical small bowel obstruction. [cite: Sutton Radiology 7e Ch 27; Bailey & Love 28e Ch 71]