A 68-year-old woman with stage 3b CKD (serum creatinine 2.1 mg/dL, eGFR 28 mL/min/1.73 m²) is admitted with acute dehydration from gastroenteritis. Her urine osmolality is 580 mOsm/kg, and fractional excretion of sodium (FE_Na) is 0.3%. After fluid resuscitation, her serum creatinine rises to 2.8 mg/dL within 24 hours, but her urine osmolality remains 620 mOsm/kg and FE_Na remains 0.2%. Which of the following best explains the rise in serum creatinine despite adequate fluid resuscitation?