What is the most likely diagnosis and urgent management for a 25‑year‑old man, post‑appendectomy, with a 10‑hour history of acute sharp left lower quadrant abdominal pain aggravated by movement and not relieved by passing flatus?

Acute Left Lower Quadrant Pain in a 25-Year-Old Post-Appendectomy Male

Order CT abdomen and pelvis with IV contrast immediately—this is the mandatory first-line test because clinical examination alone misdiagnoses 34–68% of cases, and the imaging will definitively identify diverticulitis, recurrent appendicitis (including left-sided appendix from undiagnosed situs inversus or malrotation), or other surgical emergencies. 1, 2

Most Likely Diagnosis

Acute sigmoid diverticulitis is the leading diagnosis in adults with left lower quadrant pain, even at age 25, though less common in this age group. 1, 2 However, left-sided appendicitis from situs inversus or midgut malrotation must be excluded—these are well-described causes of left lower quadrant pain that are frequently missed on initial evaluation. 3, 4, 5

Critical Diagnostic Considerations

• The history of appendectomy 4 years ago does NOT rule out appendicitis—if the prior surgery was for right-sided pain and the patient has undiagnosed situs inversus or malrotation, a left-sided appendix may still be present. 3, 4, 5

• Pain aggravated by movement, localized sharp tenderness, and progression over hours are classic features that occur in both diverticulitis and appendicitis. 1, 5

• Passing minimal gas without pain relief argues against simple bowel obstruction or functional causes and supports an inflammatory process. 1

• Pain waking the patient from sleep indicates progression to peritoneal irritation, which may signal impending perforation or abscess formation. 1, 5

Urgent Diagnostic Workup

Immediate Laboratory Tests

• Obtain C-reactive protein (CRP), white blood cell count with differential, and basic metabolic panel. 1, 2
  • CRP >50 mg/L combined with left lower quadrant tenderness and absence of vomiting has 97% accuracy for diverticulitis (though this triad occurs in only 24% of patients). 1
  • CRP >170 mg/L predicts severe diverticulitis requiring surgery or drainage with 87.5% sensitivity and 91.1% specificity. 1

Imaging Strategy

• CT abdomen and pelvis with IV contrast is rated 8/9 (usually appropriate) by the American College of Radiology and achieves 98–99% diagnostic accuracy. 1, 2

• CT will immediately reveal:

  • Sigmoid diverticulitis (pericolonic fat stranding, bowel wall thickening, abscess). 1
  • Situs inversus or malrotation with left-sided appendicitis (inflamed appendix in left lower quadrant). 3, 4, 5
  • Alternative diagnoses: epiploic appendagitis, colitis, inflammatory bowel disease, perforated colon cancer, urolithiasis, or psoas abscess. 1, 2, 6

• Do NOT rely on clinical examination alone—the classic triad of left lower quadrant pain, fever, and leukocytosis is present in only ~25% of diverticulitis cases, and clinical diagnosis has a 34–68% misdiagnosis rate. 1, 2

Common Pitfalls to Avoid

• Do not assume the prior appendectomy rules out appendicitis—verify the surgical history and confirm which side was operated on; case reports document left-sided appendicitis misdiagnosed as gastroenteritis in patients with situs inversus. 4, 5

• Do not order plain radiographs—they have extremely limited diagnostic value and will delay definitive diagnosis. 1, 2

• Do not order ultrasound as the primary test—it is operator-dependent, limited by body habitus and bowel gas, and has lower sensitivity than CT for detecting complications. 1, 2

• Do not discharge the patient without imaging if pain is progressive or associated with peritoneal signs—this patient's worsening pain with core engagement suggests evolving peritonitis. 1, 5

Management Algorithm Based on CT Findings

If CT Shows Uncomplicated Diverticulitis

• For immunocompetent patients: conservative management WITHOUT antibiotics (clear liquid diet, oral analgesics, outpatient follow-up in 48–72 hours). 1, 2

• For immunocompromised or elderly patients: antibiotics for maximum 7 days (Ertapenem 1 g q24h or Eravacycline 1 mg/kg q12h). 1, 2

If CT Shows Complicated Diverticulitis

• Small abscess (<4 cm): antibiotics alone for 7 days without drainage. 1, 2

• Large abscess (≥4 cm): percutaneous CT-guided drainage PLUS antibiotics for 4 days. 1, 2

• Perforation with free air or diffuse peritonitis: emergency surgical consultation for resection. 1, 2

If CT Shows Left-Sided Appendicitis (Situs Inversus or Malrotation)

• Immediate surgical consultation for laparoscopic appendectomy—delayed diagnosis increases the risk of perforation, abscess formation, and prolonged hospital stay. 4, 5

• Verify situs inversus on CT (heart on right side, liver on left, stomach on right)—this will be immediately apparent on imaging. 3, 4

If CT Shows Alternative Diagnosis

• Epiploic appendagitis: conservative management with NSAIDs, self-limited condition. 6, 7

• Urolithiasis: hydration, analgesia, urology referral if stone >5 mm or obstructing. 2, 6

• Colitis or inflammatory bowel disease: gastroenterology referral, disease-specific therapy. 1, 6

Red Flags Requiring Emergency Department Evaluation

• Fever, inability to tolerate oral intake, signs of peritonitis (guarding, rebound tenderness, rigidity), vomiting, bloody stools, or signs of shock mandate immediate emergency department transfer. 1, 2

• This patient's progressive pain waking him from sleep and pain with core engagement suggests evolving peritonitis—do not delay imaging or emergency evaluation. 1, 5