A 53-year-old man with gout for many years usually experiences 4-5 attacks a year. He treats each flare with an over-the-counter nonsteroidal anti-inflammatory drug (NSAID), but is not taking any medications for gout prophylaxis.On physical examination, there are no active joints presently, but he does have some tophaceous deposits on his left hand. As part of his complete evaluation, screening for renal complications of gout is performed. Which of the following findings is most likely compatible with chronic gouty nephropathy?