Mass of minerals and organic matter that may form in a kidney. Urine contains many salts in solution, and low fluid volume or high mineral concentration can cause these salts to precipitate and grow, forming stones. Large stones can block urine flow, be a focus for infection, or cause renal colic (painful spasms). They can obstruct the urinary system at various points. Treatment deals with any underlying problem (e.g., infection or obstruction), tries to dissolve stones with drugs or ultrasound (lithotripsy), or removes large ones surgically.

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