A 62-year-old man with a history of colorectal cancer presents to the emergency department with acute onset abdominal distension, vomiting, and absolute constipation for 36 hours. On examination, he has a distended, tympanic abdomen with reduced bowel sounds. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels and a transition zone in the left lower abdomen. What is the investigation of choice to confirm the diagnosis and guide further management?

Explanation

## Diagnosis and Investigation Selection The clinical presentation and plain radiograph findings are consistent with **mechanical small bowel obstruction**, likely secondary to adhesions or recurrence in a post-cancer patient. While plain radiography identified the obstruction, CT is the gold standard for confirming the diagnosis, determining the level and cause of obstruction, and assessing for complications. ### Why CT Abdomen and Pelvis with IV Contrast is Correct **Key Point:** CT with IV contrast is the investigation of choice for mechanical bowel obstruction because it: - Confirms the diagnosis with 95%+ sensitivity and specificity - Identifies the exact level of obstruction (proximal small bowel, distal small bowel, or colon) - Determines the etiology (adhesions, malignancy, volvulus, intussusception, hernia) - Assesses for complications (strangulation, perforation, ischemia) - Guides clinical decision-making (conservative vs. surgical management) **Clinical Pearl:** In a patient with prior abdominal surgery and malignancy, CT helps differentiate adhesive obstruction (often managed conservatively) from malignant obstruction (may require urgent surgery). Oral contrast is typically avoided in acute obstruction due to aspiration risk; IV contrast alone is sufficient. **High-Yield:** CT imaging has become the imaging modality of choice in modern practice for acute mechanical bowel obstruction, superseding barium studies in most centers. ### Plain Radiograph Limitations While plain films identified the obstruction, they cannot reliably: - Determine the exact cause - Assess for strangulation or ischemia - Evaluate for complications - Guide surgical planning if needed