A two-year-old with a tumor of the kidney most likely has Wilms tumor. This aggressive lesion (now with a 90% survival rate ) is microscopically interesting because it contains embryonic glomerular and tubular structures embedded in a spindle cell stroma which may also contain smooth muscle, striated muscle, bone, cailage, fat, and fibrous tissue. Abundant clear cells are a feature of renal cell carcinoma. Eosinophilic cells packed with mitochondria suggests oncocytoma. Hamaomatous blood vessels suggest angiomyolipoma. 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.