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Gout is caused by too much uric acid in the blood (hyperuricemia). Hyperuricemia usually does no harm. But sometimes when uric acid levels in the blood are too high, uric acid forms crystals that build up in the joints. The crystals can cause a gout attack. A gout attack typically causes pain, swelling, redness, and warmth (inflammation) in a single joint, most often the big toe.