False-colour X-ray image of the human pelvis, showing mulitple, bulky stones in the urinary bladder (centre, dark circular areas). Urinary stone disease (urolithiasis) describes the formation of crystalline masses along the urinary tract. Urolithiasis occurs in affluent and deprived populations. In affluent societies, high protein & carbohydrate diets are incriminated in the formation of calcium oxalate stones, whereas protein deprivation leads to stone formation in underdeveloped regions. Large stones, which may block and infect the urinary tract, may require surgical removal. Small stones may be passed out in urine.