Homogentisic acid is a chemical compound that plays a role in the metabolism of the amino acids phenylalanine and tyrosine. It is produced during the breakdown of these amino acids and is typically found in small amounts in the body. Elevated levels of homogentisic acid can lead to a condition known as alkaptonuria, which is characterized by darkening of urine and potential joint problems. In alkaptonuria, the body lacks the enzyme needed to break down homogentisic acid, causing it to accumulate. This condition is inherited in an autosomal recessive manner, meaning both parents must carry the gene for a child to be affected. Over time, the buildup of homogentisic acid can lead to complications such as arthritis and heart issues.