Gout is a painful inflammation of a joint caused by the build-up of crystals within the joint. The crystals are made of uric acid, a substance found in many foods, and usually excreted by the kidneys.
People who produce too much uric acid, or whose kidneys do not excrete it enough, are pre-disposed to the development of gout. When the levels of uric acid in the blood are high enough, it beings to collect as crystals in joints, leading to pain and swelling.
Gout most often affects the base of the big toe, but can affect any joint, and tend to recur in repeated attacks of acute inflammation.