A 28-year-old man from Mumbai presents to the nephrology clinic with polyuria (8–9 L/day) and polydipsia. Serum sodium is 148 mEq/L, and urine osmolality is 150 mOsm/kg despite high serum osmolality. MRI brain shows a sellar mass. A water deprivation test shows no improvement in urine osmolality, but administration of desmopressin (DDAVP) reduces urine output and increases urine osmolality to 600 mOsm/kg. Which segment of the nephron is PRIMARILY responsible for the clinical improvement seen after DDAVP administration?