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A kidney stone is a solid piece of material that forms from crystallization of excreted substances in the urine. The stone may remain in the kidney or break loose and travel down the urinary tract. A small stone may pass all of the way out of the body, but a larger stone can get stuck in a ureter, the bladder, or the urethra. This may block the flow of urine and cause great pain.
A kidney stone may be as small as a grain of sand or as large as a pearl, and some are as big as golf balls. They may be smooth or jagged, and are usually yellow or brown in color.
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