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 forms only 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. prevention preventative measures depend on the type of stones. in those with calcium stones, drinking plenty of fluids, thiazide diuretics and citrate are effective as is allopurinol in those with high uric acid levels in urine. dietary measures