An 18yo man presents to his GP with thirst and polyuria

An 18yo man presents to his GP with thirst and polyuria. Some 6m ago he had a significant head
injury as the result of a RTA. He is referred to the local endocrine clinic. Which of the following
results would be the most useful in confirming the dx of diabetes insipidus after a water
deprivation test (without additional desmopressin)?

a. Plasma sodium of 126mmol/l b. Plasma sodium of 150mmol/l
c. Plasma osmolality of 335mosmol/kg and urine osmolality of 700mosmol/kg
d. Plasma osmolality of 280mosmol/kg and urine osmolality of 700mosmol/kg
e. Plasma osmolality of 335mosmol/kg and urine osmolality of 200mosmol/kg

Ans. The key is E. Plasma osmolality of 335 mosmol/kg and urine osmolality of 200 mosmol/kg. [In DI plasma osmolality will be high due to excess fluid loss with urine and urine osmolality will be low due to polyuria. Hence E. is the best option here(normal plasma osmolality 275-295 mosmol/kg and normal urine osmolality is 300-900 mosmol/kg)].