A 65-year-old man, with a 45-pack-per-year history of smoking, presents with hematuria and flank pain. He reports no fever, chills, or dysuria, but he has lost 15 lb. On examination the abdomen is soft, no mass is felt, and there is no flank tenderness on percussion. His hemoglobin (Hb) is 18.5 g/dL, and his liver enzymes are normal. A CT scan of the abdomen reveals a mass in the left kidney with involvement of the renal vein. Which of the following is the most likely diagnosis?