<prompt> A 33 year old lady presents with polydipsia and polyuria. Her symptoms staed soon after a road trafic accident 6 months ago. The blood pressure is 12080 mm Hg with no postural drop. The daily urinary output is 6-8 liters. Investigation showed, Na 130 mEqL, K.3.5 mEqL, urea 15 mgdL, sugar-65 mgdL. The plasma osmolality is 268 mosmolL and urine osmolality 45 mosmolL. The most likely diagnosis is:Central diabetes insipidusNephrogenic diabetes insipidusResolving acute tubular necrosisPsychogenic polydipsia  <response>: Answer is D Psychogenic polydipsia : All of the options provided may present with polyuria polydipsia but only Psychogenic polydipsia presents with associated decreased serum and urine osmolality. Laboratory diagnostic parameters amongst conditions provided. Diabetes insipidus Nephrogenic central Resolving ATN Diuretic phase of renal failure Psychogenic polydypsia Serum osmolarity N or 1 Urinary osmolarity 1 Serum osmolarity ed ed serum Na Urinary osmolarity .fed Serum osmolarity fed Urine osmolarity Laboratory parameters for patient in question Parameter Normal value Patient in question Inference Serum Sodium Na 136-145 meqL 130 meq1 Decreased Serum potassium K 3.5-5.0 meqL 3.5 meqI Normal Serum Osmolality 285-295 mmolkg 208 mosmkg Decreased Urine Osmolality 350-1000 mosmolkg 45 mosmL Decreased. Thus the only ble answer amongst the options that satisfies decreased serum and urine osmolality is psychogenic polydypsia. Psychogenic polydipsia: Psychiatric disturbance characterized by excessive intake of water Plasma sodium and osmolality fall and urine produced is appropriately dilute Vasopressin levels become almost undetectable Characteristically the diagnosis is made by a water deprivation test. The initially low urine molarity gradually increases with the duration of water deprivation. 