Because many of its signs and symptoms are nonspecific, the differential diagnosis of acute appendicitis is extensive and includes virtually all possible abdominal sources of pain, as well as some nonabdominal sources (see table below). However, some diagnoses are more likely than others in certain patient groups. For instance, in young males with a suggestive history and physical examination, acute appendicitis is the most likely cause of right lower quadrant pain. Meckel’s diverticulitis causes similar symptoms, but is relatively uncommon. Gastroenteritis is considerably more common and should be expected when nausea and vomiting precede the abdominal pain, or when diarrhea is a prominent symptom. Crohn’s disease affecting the terminal ileum may resemble appendicitis in its initial presentation, but on further questioning, the patient typically describes a subacute course including fever, weight loss, and pain.