Q. A 4-month-old male infant is brought to the pediatrician by his parents for evaluation of excessive thirst and increased urination. The parents report that their son has been drinking more than 4L of fluids per day and urinating more than 10 times per day. He is currently at the 75th percentile for weight and his height is at the 50th percentile for his age. His physical examination is normal with no signs of dehydration. His blood glucose is normal and urine dipstick is negative for glucose, protein, and blood. The pediatrician orders a water deprivation test. The results show an inability to concentrate urine, with a specific gravity of less than 1.005 despite severe water deprivation. The administration of desmopressin does not change the results. What is the most likely cause of these signs and symptoms?