Tophus

A tophus (Latin: "stone", plural tophi) is a deposit of uric acid crystals, in the form of monosodium urate crystals, in people with longstanding hyperuricemia (high levels of uric acid in the blood). Tophi are pathognomonic for the disease gout. Most people with tophi have had previous attacks of acute arthritis, eventually leading to the formation of tophi. Chronic tophaceous gout is known as Harrison Syndrome.