While bathing her newly received 2-year-old son, a foster mother feels a mass in his abdomen. A thorough medical evaluation of the child reveals aniridia, hypospadias, horseshoe kidney, and hemihypertrophy. Which of the following is the most likely diagnosis for this child?

An abdominal mass is palpated in 85% of patients with Wilms tumor; abdominal pain is present in 40%, hypertension in about 60%, and hematuria in 12% to 24%. Because of the association of hemihypertrophy and aniridia with Wilms tumor, children with these findings should be followed with periodic physical examinations and abdominal sonograms, especially during their first 5 years. Wilms tumor and aniridia are associated with abnormalities on chromosome 11.Neuroblastoma should also be considered in the differential diagnosis of abdominal mass, especially if fever, irritability, bone pain, limp, and diarrhea are present; in the case presented, however, the other features such as aniridia and horseshoe kidney make this diagnosis less likely than Wilms tumor.