Gout is caused by abnormal metabolism of substances called purines that result in the accumulation of uric acid in the blood stream. Purines are a by-product of cell break down. When the excretion of the uric acid is hampered the accumulated uric acid in the blood stream causes crystalline deposits to form in joints or in the soft tissues. When this happens, there is a sudden onset of extreme pain with associated swelling, redness, and increased warmth to the skin or joint. Classic gout occurs in the big toe joint. It also commonly occurs in the knee joint. Rarely is it seen in more than one joint at a time. Uric acid accumulation in other joints and areas of soft tissue is less common. When gout presents in these areas it, may not be recognized as gout by the treating doctor.

Source: Kirk A. Koepsel, D.P.M.