Obstructed kidney. X-ray of the obstruction of a kidney, highlighted by the injection of a contrast medium (white). The contrast medium has revealed the kidney (branching structure, upper right), but has not passed downwards into the ureter. This is the tube that drains urine from each kidney to the bladder (not seen). An obstruction can be caused by an object like a kidney stone, or a response to infection. In children, it is usually a congenital (present from birth) defect. The blockage results in swelling of the affected kidney. It is treated by surgical repair of the affected area. The spine is seen running down far left.