A 32-year-old man from Rajasthan presents to the emergency department with severe dehydration following 3 days of diarrhea. His serum osmolality is 310 mOsm/kg, urine osmolality is 850 mOsm/kg, and urine output is 0.3 mL/kg/hr. Physical examination reveals dry mucous membranes and skin turgor loss. Which segment of the nephron is primarily responsible for generating the osmotic gradient that allows this concentrated urine formation?