A 52-year-old woman with chronic kidney disease (CKD stage 3b, eGFR 38 mL/min/1.73m²) and hypertension is evaluated for persistent polyuria and nocturia despite adequate blood pressure control. Her serum sodium is 142 mEq/L (normal 135–145), serum osmolality is 298 mOsm/kg, and random urine osmolality is 280 mOsm/kg (inappropriately dilute). Renal ultrasound shows normal-sized kidneys with mild cortical thinning. Which aspect of the countercurrent mechanism is most likely impaired in this patient, and what is the pathophysiological consequence?