What is the histologic hallmark of acute appendicitis?

• A. Lymphocytic infiltration of the mucosa
• B. Eosinophilic infiltration of the submucosa
• C. Granulomatous inflammation
• D. Neutrophilic infiltration of the muscularis propria

Answer: (D)

The key histologic feature of acute appendicitis is acute neutrophilic inflammation of the appendiceal wall, especially neutrophils invading the muscularis propria (transmural inflammation). This reflects the rapid, acute nature of the process as the inflammation spreads through the wall. Other patterns—lymphocytic infiltration of the mucosa, eosinophilic infiltration of the submucosa, or granulomatous inflammation—are associated with other conditions or chronic processes and do not represent the hallmark of acute appendicitis. So neutrophilic infiltration of the muscularis propria best captures the classic histology.