MSFinals-6580

A 50-year-old man presents to his GP with complaints of left flank pain. He has a history of smoking a pack of cigarettes a day for 25 years. On physical examination, there is tenderness in the left flank but no palpable mass. Urine dipstick testing reveals the presence of blood. A CT scan of the abdomen shows a complex cystic mass with solid and liquid components, arising from the parenchyma of the left kidney and with septations. What is the most probable diagnosis?