A 38-year-old woman with a 10-year history of diabetes mellitus type 2 presents with progressive renal dysfunction (eGFR 35 mL/min/1.73 m²). Her serum osmolality is 295 mOsm/kg (normal). She is given a water deprivation test followed by desmopressin (synthetic ADH). Her urine osmolality increases from 280 mOsm/kg (after water deprivation alone) to 620 mOsm/kg (after desmopressin). Which of the following best explains why her maximum urine osmolality is significantly lower than the normal range of 800–1200 mOsm/kg?