Answer: Acute appendicitis in appendix with sessile serrated adenoma and appendiceal diverticulum

Histology: Section of the tip shows a small sessile serrated adenoma filling the lumen. The surrounding wall shows gradual thinning leading to complete loss of muscularis propria with the mucosa extruding into periappendiceal soft tissue, consistent with an appendiceal diverticulum. The inside of the diverticulum is lined by ulcerated and inflamed colonic mucosa with focal extravasated acellular mucin. The surrounding soft tissue is edematous and inflamed. The serosal surface is also inflamed and is covered by a fibrnous exudate but is free of mucin. No perforation is identified. The adenoma is limited to the luminal mucosa.

Discussion: Sessile serrated adenoma is rare in the appendix, however, the majority of cases have been described in patients 30 years or older whose entire appendix was submitted for histologic evaluation. Appendiceal diverticula are also rare and can present with acute diverticulitis with or without perforation. Perforated diverticula may also mimic ruptured low grade appendiceal mucinous neoplasms since atypical colonic epithelium may be seen on the surface with associated extravasations of mucin. The correct diagnosis can be made by recognizing the presence of a diverticulum and by the presence of reactive rather than adenomatous epithelial atypia.