Crossing Into the Unknown. A Peculiar Cause of Ureteropelvic Junction Obstruction in a 14-Year-Old Boy

Ureteropelvic junction obstruction (UPJO) is a common cause of upper urinary tract obstruction and consequent hydronephrosis. We present a case of right UPJO caused by a crossing vessel suspected to be the right second lumbar vein. While crossing vessels are a common cause of UPJO in older children and adults, they are usually attributed to lower pole vessels. We believe that this is the first case report of UPJO caused by a lumbar vein. Recognition of crossing vessels requires knowledge of variant vasculature anatomy around the kidneys. Due to this unusual crossing vessel origin, our case likely represented an anatomical variant that would be of interest to both fields of urology and radiology.