Q. A 45-year-old male presents to the clinic with a complaint of left flank pain and bloody urine. He reports that the pain has been present for several months and has been increasing in intensity. On physical examination, the patient has a palpable mass in the left flank region and a slightly elevated serum creatinine level of 1.3 mg/dL. Imaging studies show a large, complex cystic mass in the left kidney with surrounding fat stranding and a few small enhancing nodules. Which of the following is the most likely diagnosis?