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Acute appendicitis is the most common cause of acute abdomen requiring emergency surgery in the United States, with a lifetime risk of approximately 7%. Prompt diagnosis is critical as perforation risk increases significantly after 24-72 hours of symptom onset.
Appendicitis begins with luminal obstruction — most commonly by a fecalith, lymphoid hyperplasia, or rarely a neoplasm — causing increased intraluminal pressure, mucosal ischemia, bacterial overgrowth, and eventual transmural inflammation. Perforation leads to localized abscess or generalized peritonitis.
Classic presentation is periumbilical pain migrating to the right lower quadrant (McBurney's point), accompanied by anorexia, nausea, and low-grade fever. The Alvarado score integrates symptoms, signs, and labs (leukocytosis) to risk-stratify. CT abdomen/pelvis has high sensitivity and specificity; ultrasound is preferred in children and pregnant patients to avoid radiation. Key differentials include ovarian pathology in women, Meckel's diverticulitis, mesenteric adenitis, and Crohn's ileitis.
Key imaging focus: Dilated appendix >6mm, periappendiceal fat stranding, appendicolith