refers to the bluish-black discoloration of certain tissues, such as the ear cartilage, the ocular (eye) tissue, and other body locations when it is due to exposure to various substances. It has been reported most commonly with topical application of hydroquinones to the skin. The discoloration may be caused by an effect on tyrosinase (an enzyme located in melanocytes, which are skin cells that produce pigment), or by inhibiting homogentisic acid oxidase, resulting in the accumulation and deposition of homogentisic acid (HGA) in cartilage. The discoloration is often permanent, but when exogenous ochronosis is caused by topical hydroquinones, carbon dioxide lasers and dermabrasion have been reported to be helpful. Exogenous ochronosis is different from hereditary ochronosis, which is an inherited condition that occurs with alkaptonuria. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.