A patient with polyuria and hydronephrosis: question

A 5-year-old boy was referred for treatment due to a longstanding history of polyuria/polydipsia of more than 3 liters per day and failure-to-thrive. He woke up three to four times during the night to drink. The mother reported feeding problems, including vomiting since he was 5 months old. The results of biochemical tests at 13 months of age had been normal, with plasma sodium of 143 mmol/l and creatinine of 36 μmol/l. His family history was remarkable for a 1-year-old brother having similar symptoms. In addition, a maternal uncle had a longstanding history of polyuria. Physical examination was unremarkable. The boy had a healthy appearance, his height and weight were at the 2nd percentile and blood pressure was 74/52 mmHg. Repeat renal function biochemistry results were all in the normal range, with sodium at 142 mmol/l and creatinine at 43 μmol/l. Urine albumin was <3 mg/l. The patient underwent a water deprivation test, leading to a maximum urine osmolality of 269 mOsm/kg with a concomitant plasma osmolality of 305 mOsm/kg. A subsequent dose of 0.3 mcg DDAVP given intravenously failed to increase the urine osmolality further. A renal ultrasound was obtained, which is depicted in Fig. 1.