Soru

A $7$-year-old boy is brought to the clinic by his parents due to a $2$-month history of bedwetting. He had been completely toilet trained during the day and night since the age of $4$. His parents note that he now frequently asks to use the bathroom during school and constantly craves cold water. The patient has no history of head trauma or recent illnesses. His temperature is $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure is $95/60 \text{ mm Hg}$, and pulse is $98\text{/min}$. Physical examination is unremarkable, and his growth is tracking along the $50^{\text{th}}$ percentile for both height and weight.

Initial laboratory results are as follows:

A formal water deprivation test is performed. After $6$ hours, his urine osmolality remains low at $120 \text{ mOsm/kg H}_2\text{O}$, and his serum sodium rises to $152 \text{ mEq/L}$. Subcutaneous desmopressin is then administered. Two hours later, his urine osmolality increases to $580 \text{ mOsm/kg H}_2\text{O}$.

Which of the following is the most appropriate next step in the management of this patient?