AKU is a rare hereditary autosomal recessive metabolic disorder caused by pathogenic variants in the HGD gene, resulting in HGD enzyme deficiency and HGA accumulation. The consequent elevation of HGA level is either excreted in urine or oxidized via a benzoquinone acetic acid to form a melanin-like pigment, which deposits in connective tissues in a process known as ochronosis. The gene discussed is HGD; the disease is alkaptonuria.