A 62-year-old man with chronic kidney disease presents with polyuria and inability to concentrate urine (urine osmolality 250 mOsm/kg despite serum osmolality of 310 mOsm/kg). Renal biopsy shows intact loop of Henle architecture but severe vasa recta atrophy and reduced medullary blood flow. Which finding best explains the loss of urine-concentrating ability despite preserved loop of Henle structure?