Case 350

History A 16-year-old girl presented to the emergency department with a 1-month history of right lower back pain. She was originally seen at an urgent care clinic, where a lumbar radiograph was reportedly normal, and she was prescribed naproxen. However, she came to the emergency department once pain had escalated to 10 out of 10. Over the past month, the patient’s right flank had increased in size and “hardened” to touch. She reported no trauma or other inciting incident. She had a prior history of laparoscopic appendectomy for perforated appendicitis more than 6 years earlier at an outside hospital. A prior contrast-enhanced abdominal CT scan, obtained 2 weeks after the appendectomy, was available for review ( Fig 1 ). At evaluation, the patient was febrile and mildly tachycardic (104 beats per minute). Examination of the right flank revealed a 4-cm area of erythema and tenderness with 10-cm surrounding induration. She had leukocytosis (white blood cell count, 12 × 10 9 /L; reference range, <9.44 × 10 9 /L) and elevated C-reactive protein (23