A 4-year-old girl has complained of abdominal pain for the past month. On physical examination, she is febrile, and palpation of the abdomen shows a tender mass on the right. Bowel sounds are present. Laboratory studies show hematuria without proteinuria. An abdominal CT scan shows a 12-cm circumscribed, solid mass in the right kidney. A right nephrectomy is performed, and the gross appearance of the mass is shown above. Which of the following is the most likely diagnosis?