Alkaptonuria is a rare genetic disorder caused by a deficiency of the enzyme homogentisate oxidase. This enzyme is essential for breaking down the amino acids tyrosine and phenylalanine. When it is lacking, a substance called homogentisic acid accumulates in the body, leading to various health issues. One of the most noticeable symptoms of alkaptonuria is the darkening of urine when exposed to air, which occurs due to the oxidation of homogentisic acid. Over time, this condition can also lead to joint problems and heart issues, as the accumulated acid can deposit in connective tissues, a process known as ochronosis.