Question

An $8$-month-old boy is evaluated in the pediatric emergency department for irritability, poor feeding, and passing copious amounts of dilute-appearing urine over the past three days. His parents report that he frequently demands breast milk or water but often vomits shortly afterward. His birth and prenatal history were unremarkable. He is currently at the $5$ th percentile for weight and $25$ th percentile for length, having dropped from the $50$ th percentile for weight at his $4$-month visit. Temperature is $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), blood pressure is $85/50 \text{ mm Hg}$, and pulse is $160/\text{min}$. Physical examination shows dry mucous membranes, a sunken anterior fontanelle, and poor skin turgor. Initial laboratory studies show:

Serum sodium: $154 \text{ mEq/L}$
Serum potassium: $4.0 \text{ mEq/L}$
Serum osmolality: $318 \text{ mOsm/kg H}_2\text{O}$
Urine osmolality: $95 \text{ mOsm/kg H}_2\text{O}$

The patient is admitted, and subcutaneous desmopressin is administered. Two hours later, a repeat urine osmolality is $105 \text{ mOsm/kg H}_2\text{O}$. Which of the following is the most appropriate long-term pharmacotherapy to reduce polyuria in this patient?