A 58-year-old man with diabetes mellitus presents with acute kidney injury. Urinalysis shows muddy brown casts and proteinuria <1 g/day. Urine sodium is 45 mEq/L and fractional excretion of sodium (FENa) is 2.1%. Which single feature best distinguishes acute tubular necrosis (ATN) from prerenal azotemia in this patient?

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