Answer is C (Water deprivation test is required) : Diagnosis of diabetes insipidus often requires a water deprivation test. Water deprivation test is used to establish the diagnosis of diabetes insipidus and to differentiate Central and Nephrogenic forms of Diabetes Insipidus. Patients with diabetes insipidus have a normal or increased plasma osmolality (> 280 mosm/l) and decreased urine osmolality (<300mosm/1). 'Water deprivation test' typically demonstrates an increase in plasma osmolality with failure of urinary osmolality to increase (no change) during H20 deprivation in cases of diabetes insipidus thereby suggesting the diagnosis. Diabetes Insipidus is associated with plasma osmolality > 280 mosm/kg and urinary osmolalitv < 300 mosm /kg In normal subjects the plasma osmolality ranges from 275-295 mosm /kg while the range of urine osmolality is wide, from 40 to 1000 mosm/kg depending on the state of hydration. After water deprivation the urine osmolality will normally rise to exceed 800 mosm/kg while the plasma osmolality remains below 295 mosm/kg (U/P >2) In Diabetes Insipidus subjects the plasma osmolality ranges from normal to high (>275 mosm/kg) while the urine osmolality is typically < 300 mosm/1. After water deprivation plasma osmolality rises to > 295 mosm/kg while urinary osmolality fails to rise appropriately (Fails to rise to >300 mosm/kg) (U/P Ratio < 2.0) Water deprivation test is often required to establish a diagnosis of Diabetes insipidus Water deprivation test is indicated in patients with hypotonic polyuria (Uosm < 300 mosm/kg) and a plasma osmolality 410 within the normal range (Posm = 275 -- 295 mosm/kg) to establish the diagnosis of diabetes insipidus and differentiate central & nephrogenic forms.