A 35-year-old man with a 2-week history of polyuria and polydipsia presents to the clinic. Serum sodium is 152 mEq/L and urine osmolality is 150 mOsm/kg after overnight fluid restriction. A water deprivation test is performed: urine osmolality remains 160 mOsm/kg. Which single feature best distinguishes Central Diabetes Insipidus from Nephrogenic Diabetes Insipidus in this patient?