A 13-year-old boy with no personal history of interest. His relevant family history included a brother who had died of Wilms' tumour. He consulted for micturition symptoms (dysuria, pollakiuria) with no other symptoms. The urine dipstick was negative. Treatment with trimethoprim-sulfamethoxazole was started until the urine culture result was reported as sterile. In the absence of improvement after one week, the patient visited the emergency department. The anamnesis was associated with micturition symptoms and anuria of 18 hours' duration, together with abdominal pain located in the hypogastrium and right iliac fossa. There was no history of surgery or fever since the onset of the symptoms. On examination, she was in good general condition with no signs of dehydration, and her vitals were within normal values. Cardiopulmonary auscultation was normal. On abdominal examination, palpation was painful in the hypogastrium and right iliac fossa, with local signs of peritoneal irritation. There was no bladder balloon. The rest of the examination showed no relevant findings.
Laboratory tests showed no findings of interest: leukocytes 5400 μl, neutrophils 2370 μl (44%), haemoglobin 13.9 mg/dl, C-reactive protein <2.9 mg/l. An abdominal X-ray was performed, which showed no abnormalities, and an abdominal ultrasound with no findings in the urinary tract, with a virtually empty bladder.
However, an 8 mm thickened appendix with periappendicular free fluid and thickening of the adjacent fat was visualised; findings compatible with acute appendicitis.

Intravenous fluid infusion was performed in an expansion regime, obtaining adequate diuresis in a short time. Given the diagnosis of acute appendicitis, he underwent surgery under general anaesthesia, identifying phlegmonous appendicitis and performing an appendectomy. The subsequent evolution was favourable, with no postoperative complications.