Which substance accumulates in alkaptonuria?

Alkaptonuria is a metabolic disorder characterized by the accumulation of homogentisic acid due to a deficiency in the enzyme homogentisate oxidase. This enzyme is involved in the breakdown of the amino acids phenylalanine and tyrosine. When homogentisate oxidase is deficient, homogentisic acid can build up in the body, leading to various symptoms, including darkening of the urine and potential joint problems due to deposits in connective tissues.

The other substances listed do not accumulate in alkaptonuria. Fumarate is an intermediate in the citric acid cycle and is not directly related to this disorder. Tyrosine is one of the amino acids involved in the metabolic pathway that leads to homogentisic acid, but it does not accumulate as a result of the enzymatic block. Phenylacetate is a compound that can appear in disorders involving phenylalanine, but it is unrelated to alkaptonuria.

Thus, the correct answer, homogentisic acid, is the substance that accumulates in the body due to the enzymatic deficiency characteristic of alkaptonuria.