<P> People with certain rare inborn errors of metabolism have a propensity to accumulate crystal - forming substances in their urine . For example, those with cystinuria, cystinosis, and Fanconi syndrome may form stones composed of cystine . Cystine stone formation can be treated with urine alkalinization and dietary protein restriction . People afflicted with xanthinuria often produce stones composed of xanthine . People afflicted with adenine phosphoribosyltransferase deficiency may produce 2, 8 - dihydroxyadenine stones, alkaptonurics produce homogentisic acid stones, and iminoglycinurics produce stones of glycine, proline and hydroxyproline . Urolithiasis has also been noted to occur in the setting of therapeutic drug use, with crystals of drug forming within the renal tract in some people currently being treated with agents such as indinavir, sulfadiazine and triamterene . </P> <P> Urolithiasis refers to stones originating anywhere in the urinary system, including the kidneys and bladder . Nephrolithiasis refers to the presence of such stones in the kidneys . Calyceal calculi are aggregations in either the minor or major calyx, parts of the kidney that pass urine into the ureter (the tube connecting the kidneys to the urinary bladder). The condition is called ureterolithiasis when a calculus is located in the ureter . Stones may also form or pass into the bladder, a condition referred to as bladder stones . </P> <P> Stones less than 5 mm (0.2 in) in diameter pass spontaneously in up to 98% of cases, while those measuring 5 to 10 mm (0.2 to 0.4 in) in diameter pass spontaneously in less than 53% of cases . </P> <P> Stones that are large enough to fill out the renal calyces are called staghorn stones, and are composed of struvite in a vast majority of cases, which only forms in the presence of urease - forming bacteria . Other forms that can possibly grow to become staghorn stones are those composed of cystine, calcium oxalate monohydrate, and uric acid . </P>

What's the average size of a kidney stone