Gout results from prolonged untreated hyperuricemia. Uric acid is the end product of purine metabolism and interestingly humans are the only mammals lacking the enzyme uricase. Gout may result from overproduction (10% of cases) or underexcretion of uric acid (90% of cases). High urate levels in plasma result in supersaturation of synovial fluid which is a poorer solvent resulting in precipitation of crystals in and around the synovium. The image shows several gouty tophi surrounded by foreign body giant cells.