Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. Ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of the disorder. This blue-black pigmentation usually appears after age 30. People with alkaptonuria typically develop arthritis, particularly in the spine and large joints, beginning in early adulthood. Other features of this condition can include heart problems, kidney stones, and prostate stones. A defect in the HGD gene causes alkaptonuria. The gene defect makes the body unable to properly break down certain amino acids (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air. Alkaptonuria is inherited, which means it is passed down through families. If both parents carry a nonworking copy of the gene related to this condition, each of their children has a 25% (1 in 4) chance of developing the disease. Symptoms are: Urine in an infant's diaper may darken and can turn almost black after several hours. However, many people with this condition may not know they have it. The disease is most often discovered in mid-adulthood (around age 40), when joint and other problems occur. Symptoms may include: Arthritis (especially of the spine) that gets worse over time, Darkening of the ear, Dark spots on the white of the eye and cornea