HGD and alkaptonuria: Alkaptonuria (AKU; MIM number 203500) is an ultrarare (1 : 250.000–1.000.000 incidence) autosomal recessive inborn error of catabolism of the aromatic amino acids due to a deficient activity of the enzyme homogentisate 1,2-dioxygenase (HGD) leading to the accumulation of homogentisic acid (2,5-dihydroxyphenylacetic acid; HGA).