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Kidney stones are hard objects that form in a person's urinary tract. About 80 percent of kidney stones are composed of calcium, mainly calcium oxalate and/or phosphate. Five percent of kidney stones are made of uric acid, 2 percent are made of the amino acid cystine, and the remainder are made of magnesium and ammonium phosphate.
About 20 percent of kidney stones are harmful, causing chronic (ongoing) urinary infections. Those stones contain a combination of calcium, magnesium, and ammonium phosphate. They form due to the alkalinity of the urine and bacterial action on urea (a substance in urine).
Sources: The American Medical Association Encyclopedia of Medicine, p. 225; Merck Manual of Diagnosis and Therapy, 15th ed., p. 1641.
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