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Enlarged kidney. Coloured 3-D ultrasound scan of a kidney swollen due to hydronephrosis. The renal pelvis (dark area, centre) of the kidney has become greatly distended with urine, due to an obstruction in the urinary tract. The obstruction may occur in the ureter that leads from the kidney to the bladder or at the bladder outflow. The obstruction may be caused by kidney stones, chronic kidney disease, or prostate gland enlargment. Ultrasound scanning is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The returning echoes are used to build an image of an internal structure. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR NAJEEB LAYYOUS)