A 68-year-old woman with a history of hypertension and chronic kidney disease stage 3a (GFR 45 mL/min/1.73 m²) is admitted with acute dehydration secondary to gastroenteritis. On admission, her serum creatinine is 1.5 mg/dL (baseline 1.4 mg/dL), serum urea is 52 mg/dL (baseline 28 mg/dL), and urine osmolality is 650 mOsm/kg. Her urine sodium is 8 mEq/L. Over the next 48 hours, after fluid resuscitation with 2 liters of normal saline, her serum creatinine falls to 1.3 mg/dL and serum urea to 30 mg/dL. Which of the following best explains the disproportionate elevation of serum urea relative to serum creatinine during the acute dehydration phase?