HGD and alkaptonuria: Alkaptonuria is a rare metabolic disorder caused by mutations in the HGO gene, leading to a deficiency in homogentisate 1,2-dioxygenase, with an incidence of 1 in 250,000 to 1 in 1,000,000.3,5 This enzyme deficiency results in homogentisic acid (HGA) accumulation, which in excess is oxidized to benzoquinones.