A 28-year-old man from Mumbai presents with polyuria (8 L/day) and polydipsia for 3 weeks. Serum osmolality is 310 mOsm/kg and urine osmolality is 150 mOsm/kg despite normal plasma ADH levels. A water deprivation test shows no change in urine osmolality, but intranasal desmopressin administration increases urine osmolality to 600 mOsm/kg. Which segment of the nephron is primarily responsible for the concentrating defect in this patient?