A 50-year-old man with advanced cirrhosis of the liver is evaluated for polyuria and nocturia. Laboratory studies reveal a blood urea nitrogen (BUN) of 5 mg/dL (normal 7–20 mg/dL), a serum sodium of 148 mEq/L, and a plasma osmolality of 305 mOsm/kg (normal 275–295 mOsm/kg). A desmopressin challenge leads to only a minimal rise in urine osmolality. A renal papillary biopsy demonstrates a markedly reduced urea concentration in the interstitial fluid of the inner medulla compared with normal. Which of the following changes most likely accounts for the decreased urea concentration in the inner medulla and the patient’s inability to maximally concentrate his urine?