Gout is a disorder of purine metabolism characterized by elevation of uric acid in the blood and episodes of arthritis due to uric acid crystal deposition in synovial fluid and the resulting inflammation. Deposition of urate in cartilage and subcutaneous tissue (as tophi) and in the kidneys and urinary tract (causing stones and renal failure) are other important effects. The arthritis is typically of sudden onset with severe pain, often affecting the great toe first and large joints in general. Treatment with allopurinol prevents recurrences.