<prompt> Central diabetes insipidus is characterised by-Low plasma and low urine osmolalityHigh plasma and high urine osmolalityLow plasma and high urine osmolalityLow urine and high plasma osmolality  <response>: Central diabetes insipidus is characterised by deficient production of ADH by the hypothalamus. In pituitary, gestational, or nephrogenic DI, there is polyuria 5-20 L or more in 24 hrsand the urine is of low specific gravity and osmolality. The polyuria results in a small 1-2 decrease in body water and a commensurate increase in plasma osmolarity and sodium that stimulates thirst and a compensatory increase in water intake. Thus in diabetes insipidus there is low osmolality of urine below 600 mOsmkg in the presence of high plasma osmolality greater than 300 mOsmkg. The most common dd is primary polydipsia where the urine is excessively dilute with low plasma osmolality. Reference : page 794 Davidsons Principles and practice of Medicine 22nd edition 