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 meq/L 130 meq/1 Decreased Serum potassium (K) 3.5-5.0 meq/L 3.5 meq/I Normal Serum Osmolality 285-295 mmol/kg 208 mosm/kg Decreased Urine Osmolality 350-1000 mosmol/kg 45 mosm/L 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.