Homogentisic acid is a chemical compound that plays a role in the metabolism of the amino acids tyrosine and phenylalanine. It is an intermediate in the breakdown of these amino acids and is produced in the body during normal metabolic processes. In individuals with a rare genetic disorder called alkaptonuria, homogentisic acid accumulates in the body due to a deficiency in the enzyme that breaks it down. This accumulation can lead to various health issues, including joint problems and darkening of the urine, as the excess acid is excreted.