Kidney stone. Sectioned human kidney diseased with calculi (stones) including a large staghorn stone. At lower centre, a staghorn calculus fills the pelvic region of the kidney; a smaller stone is at right; cavities in the tissue contained further stones. Most kidney stones are caused by precipitation from solution of substances in the urine, and consist mainly of calcium oxalate and/ or phosphate. About 20% of calculi are linked to chronic urinary tract infection. The obstruction of the urine flow caused by such stones may lead to rapid kidney damage. Smaller stones can be passed in the urine; larger stones are surgically removed, or broken down by lithotripsy.