Urinary reflux. Coloured X-ray urogram showing reflux (back-flow) of urine (red) from the bladder (red circle, bottom) to the kidneys (top right and top left). Bones of the spine and pelvis (yellow) can be seen in the background. The two yellow vessels descending from the kidneys to the bladder are the ureters, through which urine normally drains. Urinary reflux (vesico-ureteric reflux) is caused by a failure of the valves at the base of the ureters. Persistent reflux can lead to kidney damage and infection. In this patient the kidney on the left appears the most severely affected.