Late Diagnosis of X-Linked Arginine Vasopressin Resistance Presenting With Flow Uropathy in Adulthood

Congenital arginine vasopressin resistance (AVP-R) is a rare inherited disorder caused by renal unresponsiveness to arginine vasopressin (AVP), leading to persistent polyuria and polydipsia. We report a 19-year-old male individual with a long history of excessive thirst and urination who remained undiagnosed until adulthood. Despite normal growth and development, he was found to have bilateral hydronephrosis and underwent pyeloplasty at age 19. Postoperative fluid restriction resulted in life-threatening hypernatremia and seizures. Further evaluation revealed persistently low urine osmolality despite hypernatremia, elevated plasma copeptin, and no response to desmopressin or vasopressin, confirming AVP-R. Genetic testing identified a hemizygous arginine vasopressin receptor 2 (<i>AVPR2</i>) missense variant (c.357G>C; p.Gln119His), confirming X-linked congenital AVP-R. Treatment with thiazide diuretic, sodium restriction, and liberal fluid intake led to symptom relief and biochemical stability. This case highlights the diagnostic challenges of congenital AVP-R in patients without growth impairment and underscores the importance of molecular confirmation for appropriate management and family screening.