In a 60-year-old female with polydipsia, water deprivation test was performed and the values obtained are as following -Baseline: Na - 139 mEq, K-3.6mEq. Osm.plasma-274 mosm/l, Osm. urine-150 after vasopressin: Na-142mEq, K-3.9mEq, Plasma osm-271 mosm/ l,Urine osm - 489 mosm/l. These findings suggest a diagnosis of -