A large infiltrative mass is present in the right kidney with extension of mass into the renal pelvic fat, the right renal vein and IVC. There is also retroperitoneal lymphadenopathy and splenomegaly. A. Non-Hodgkin's lymphoma can involve the kidney but is seen on presentation in only 5.8% of cases. Although it can involve the kidney as a single mass, renal lymphoma most commonly presents as multiple lymphomatous masses. Additionally, renal vein and IVC invasion would be distinctly unusual for lymphoma. B. Angiomyolipoma is a benign tumor of the kidney that is characterized by regions of macroscopic fat (seen in 95% of cases). No areas of fat density are seen in the images provided with this case. Additionally, renal vein and IVC invasion and lymphadenopathy would not be a characteristic of this benign tumor. C. Renal medullary carcinoma is an unusual tumor that almost always occurs in young patients with sickle cell trait. No cases have been repoed in patients with sickle cell disease. The tumor arises from the calyceal epithelium and grows in an infiltrative pattern. It is a very aggressive tumor with early metastases to lymph nodes and vascular invasion. D. Transitional cell carcinoma can fill the renal pelvis and diffusely infiltrate the kidney as in this case. However, transitional cell carcinomas typically affect older individuals and would be rare to affect someone of this age. Also, transitional cell carcinomas would not demonstrate vascular invasion as in this case.