A 32-year-old man from Delhi presents with polyuria (8 L/day) and polydipsia for 3 months. Serum sodium is 148 mEq/L, osmolality 310 mOsm/kg. Urine osmolality is 150 mOsm/kg despite high plasma osmolality. After a water deprivation test, urine osmolality remains 160 mOsm/kg. MRI brain shows a sellar mass. Which segment of the nephron is PRIMARILY responsible for the inability to concentrate urine in this patient?

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