A

Hypernatremia with normal bicarbonate and potassium, combined with very dilute urine (specific gravity 1.005) despite serum osmolality of 295 mOsm/kg, indicates an inability of the kidneys to concentrate urine—hallmark of diabetes insipidus and the classic cause of polyuria-polydipsia in children [Vaqar et al., 2012, PMID 22630107; Cheetham & Baylis, 2002, PMID 12431131].