A 9-month-old boy is admitted to the hospital after a right upper quadrant (RUQ) abdominal mass was noted during a health supervision visit. His parents voice no concerns other than mild constipation during the past few weeks, including no abdominal distention or pain, changes in appetite, fever, weight change, or easy bruising and bleeding.

On physical examination he is active and well appearing. Vital signs are as follows: temperature, 97.9°F (36.6°C); pulse, 132 beats/min; respiratory rate, 32 breaths/min; and blood pressure, 100/75 mm Hg. Weight and weight-for-length are appropriately tracking, at the 42nd and 72nd percentiles for age, respectively. A well-demarcated, nontender RUQ mass extending from the right costal margin to 2 cm above the iliac crest and crossing midline is palpated. No splenomegaly is appreciated. Cardiopulmonary, skin, and neurologic examination findings are normal.

Initial laboratory data show a normal complete blood cell count, uric acid level, lactate dehydrogenase level,...

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