A 10-MONTH-OLD previously healthy girl presented with a 3-month history of an abdominal mass. This mass was detected at a 6-month checkup by a primary care physician. The results of liver and kidney function tests as well as of a hepatic panel were all negative. No further workup was done. The infant was brought in because of a progressive increase in the size of the mass. She was otherwise asymptomatic and was gaining weight appropriately.
The mother vaguely recalled questions of a possibly enlarged liver in the neonatal period. Examination revealed a healthy-looking, active, happy little girl who appeared well developed and well nourished. No jaundice was detected. A 13-cm, firm, nontender mass was palpable in both the upper- and lower-right quadrants. Bowel sounds were audible only in the lower-left quadrant. Results of a complete blood cell count, an electrolyte analysis, and a liver function test were normal. An abdominal