A 32-year-old Indian male with sickle cell disease (HbSS) presents with polyuria and nocturia. His serum osmolality is 295 mOsm/L, but urine osmolality remains fixed at 310 mOsm/L despite fluid restriction and desmopressin challenge. A renal ultrasound shows papillary atrophy. The structure marked **C** in the diagram (medullary interstitium) normally maintains a hyperosmotic gradient up to 1200 mOsm/L. Which of the following best explains the loss of urine concentrating ability in this patient?