A 4-year-old boy was in usual good health until presentation to his pediatrician with a 1-week history of abdominal pain, nausea, vomiting, anorexia, and intermittent fever. A right-lower-quadrant mass was palpated, and the patient was referred to the hospital with the presumptive diagnosis of appendicitis.
Physical examination at the hospital revealed a well-nourished, alert boy who was afebrile and had normal vital signs. No significant adenopathy was appreciated. There were decreased breath sounds at the right lower lung field. The abdomen was distended, and a mass approximately 17×8 cm was palpated.
Laboratory values were significant for a blood urea nitrogen value of of 7.15 mmol/L; creatinine, 220 μmol/L; lactate dehydrogenase, 4098 U/L; and uric acid, 1060 μmol/L. His white blood cell count was 8.1×109/L, with a normal differential. Protein in the urine measured 1+, and many uric acid crystals were present.
The roentgenographic evaluation was begun with an