A 68-year-old man with a 3-day history of left lower quadrant pain, fever (38.5°C), and leukocytosis (WBC 14,000/μL) undergoes contrast-enhanced CT abdomen/pelvis. The imaging shows sigmoid diverticula with pericolonic fat stranding. The structure marked **C** in the diagram demonstrates bowel wall thickening with phlegmon formation. Which of the following best describes the pathophysiologic mechanism underlying this radiologic finding?

Explanation

## Why "Micro-perforation of an obstructed diverticulum leading to localized inflammation and edema of the colonic wall" is right The clinical anchor defines acute diverticulitis as inflammation of colonic diverticula caused by **micro-perforation of an obstructed diverticulum**. The bowel wall thickening with phlegmon (marked **C**) is the direct radiologic manifestation of this inflammatory process. Phlegmon represents a localized area of inflammation and edema in the colonic wall and surrounding tissues, which develops when bacteria and inflammatory mediators escape the obstructed diverticulum. This is the pathophysiologic foundation of uncomplicated acute diverticulitis as described in Sabiston Surgery 21e and Cameron Current Surgical Therapy 13e. ## Why each distractor is wrong - **Transmural necrosis of the colonic mucosa due to vascular insufficiency**: This describes ischemic colitis, not diverticulitis. While ischemic colitis can present with left lower quadrant pain and bowel wall thickening, it is not caused by diverticular micro-perforation and is a separate differential diagnosis. The presence of diverticula on imaging makes diverticulitis the primary diagnosis. - **Stricture formation from chronic fibrosis of the muscularis propria**: Strictures are a late complication of chronic or recurrent diverticulitis, not the acute mechanism. Strictures develop over weeks to months after repeated episodes, whereas phlegmon formation occurs acutely within days of the initial micro-perforation. - **Mucosal ulceration secondary to dysbiosis and altered intestinal microbiota**: While dysbiosis may contribute to diverticulosis risk, it does not directly explain the acute inflammatory response and phlegmon formation seen in active diverticulitis. The primary trigger is mechanical obstruction and micro-perforation, not primary mucosal ulceration. **High-Yield:** Phlegmon in acute diverticulitis = localized inflammation from micro-perforation; contrast-enhanced CT is the gold standard (95% sensitivity, 99% specificity) for diagnosis. [cite: Sabiston Surgery 21e; Cameron Current Surgical Therapy 13e]