A 42-year-old man from Rajasthan presents to the emergency department with severe dehydration after 3 days of diarrhea and minimal fluid intake. His serum osmolality is 310 mOsm/kg (normal 280–295), and urine osmolality is 850 mOsm/kg. His urine output is 400 mL/day. On examination, he has dry mucous membranes and poor skin turgor. Which segment of the nephron is primarily responsible for generating the osmotic gradient that allows him to concentrate his urine to this degree?