A kidney stone is simply a mineral deposit that forms in the urinary tract. Kidney stones develop when crystals, which are present in the urine, begin to stick together and gradually build up into a rock-like mass. Initially, kidney stones are quite small in size, but over time, as more and more crystals attach, they can grow quite large. The medical term for this condition is nephrolithiasis, or renal stone disease. The kidneys filter waste products from the blood and add them to the urine that the kidneys produce. When waste materials in the urine do not dissolve completely, crystals and kidney stones are likely to form. Small stones can cause some discomfort as they pass out of the body. Regardless of size, stones may pass out of the kidney, become lodged in the tube that carries urine from the kidney to the bladder and cause severe pain that begins in the low back and radiates to the side or groin. A lodged stone can block the flow of urine, causing pressure to build in the affected ureter and kidney. Increased pressure results in stretching and spasm, which cause severe pain. Kidney stones form when there is a high level of calcium, oxalate, or uric acid in the urine; a lack of citrate in the urine; or insufficient water in the kidneys to dissolve waste products. The kidneys must maintain an adequate amount of water in the body to remove waste products. If dehydration occurs, high levels of substances that do not dissolve completely may form crystals that slowly build up into kidney stones. Urine normally contains chemicals that prevent the formation of crystals. Low levels of these inhibitors can contribute to the formation of kidney stones.