Robert L. Lebowitz, M.D. Children' s Hospital, Harvard Medical School Boston, Massachusetts

This boy fell while playing basketball and had a CT scan. It showed left hydronephrosis, so he was referred to Children's Hospital. An ultrasound exam showed a horseshoe kidney with a normal right side and severe hydronephrosis on the left (Figure 1). An IVP confirmed the presence of the horseshoe kidney and showed a normal, nonrotated right kidney with an anterior UP junction and only faint visualization of the left kidney (Figure 2). A DMSA scan showed 13% function on the left (Figure 3). A retrograde ureterogram suggested UP obstruction from a crossing vessel (Figure 4). He underwent dismembered pyeloplasty and a crossing vessel was found to be the culprit. It was causing extrinsic obstruction just below the UP junction.