A 22-year-old male presents to the emergency department with a 12-hour history of periumbilical pain that has now migrated to the right lower quadrant. He has anorexia, nausea, and low-grade fever (38.2°C). On examination, he has tenderness at McBurney's point with rebound tenderness. Ultrasound is performed using graded compression technique. The structure marked **A** in the diagram is identified as a non-compressible blind-ended tubular structure measuring 8 mm in outer diameter. Based on this ultrasound finding, what is the most likely diagnosis?

Explanation

## Why Acute appendicitis is right The non-compressible blind-ended tubular structure with outer diameter > 6 mm (in this case 8 mm) is the pathognomonic ultrasound finding for acute appendicitis. According to Rumack Diagnostic Ultrasound 6e, the graded compression technique of Puylaert identifies the normal appendix as compressible and < 6 mm in diameter; when the appendix is non-compressible and dilated > 6 mm, it indicates appendiceal distention due to luminal obstruction and inflammation. Combined with the classic clinical presentation (periumbilical pain migrating to RLQ, McBurney tenderness, rebound tenderness, fever, and leukocytosis), this finding is diagnostic. Acute appendicitis is the most common acute surgical abdomen worldwide with a lifetime risk of 7–8% and peak incidence at 10–30 years, making it the most likely diagnosis in this age group with this presentation. ## Why each distractor is wrong - **Mesenteric adenitis**: While it can mimic appendicitis clinically, it does not produce a dilated non-compressible tubular structure > 6 mm. Instead, it presents with enlarged mesenteric lymph nodes. The appendix remains normal in caliber and compressible. - **Meckel's diverticulitis**: Although it can present similarly to appendicitis, Meckel's diverticulum is a remnant of the vitelline duct located on the antimesenteric border of the ileum (typically 2 feet from the ileocecal valve), not in the right lower quadrant where the appendix is located. The ultrasound finding of a dilated non-compressible structure at the typical appendiceal location rules this out. - **Cecal diverticulitis**: Diverticulitis of the cecum presents with inflammation of the cecal wall but does not produce the characteristic blind-ended non-compressible tubular structure > 6 mm that is specific to appendicitis. The cecum is a larger, sacculated structure, not a tubular blind pouch. **High-Yield:** Non-compressible appendix > 6 mm on ultrasound with graded compression is the gold standard finding for acute appendicitis; normal appendix is compressible and < 6 mm. [cite: Rumack Diagnostic Ultrasound 6e Ch 8; Sabiston Surgery 21e Ch 50]