A pediatrician discovers a large mass in the abdomen of a 3 year old child. Ultrasound examination demonstrates that the mass appears to arise from the right kidney. Which of the following tumours is most likely present?

Nephroblastoma (Wilms tumor) characteristically affects children between 2 and 4 years of age and can form large spherical masses composed of variegated tissues including primitive renal epithelial elements, a sarcomatous-appearing stroma, aboive glomeruli and tubules, and heterologous tissues such as muscle, cailage, fat, and bone. Coical adenoma is a small (under 2 cm) benign tumor that is usually found incidentally at autopsy. Hemangiomas can occur in the kidney, but would not usually produce a large mass. Oncocytomas are benign tumors that can be large but do not usually affect young children. Ref: Graham D.K., Quinones R.R., Keating A.K., Maloney K., Foreman N.K., Giller R.H., Greffe B.S. (2012). Chapter 31. Neoplastic Disease. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.