A 28-year-old male presents to the nephrology clinic with a 3-month history of polyuria (8–10 L/day) and polydipsia. He reports that despite adequate fluid intake, he remains persistently thirsty. Serum osmolality is 310 mOsm/kg (normal: 280–295), and urine osmolality is 150 mOsm/kg (normal: 300–900). Plasma ADH level is elevated at 45 pg/mL (normal: 2–12). A water deprivation test shows no change in urine osmolality. Which segment of the nephron is MOST likely damaged, and what is the underlying pathophysiology?