HGD and alkaptonuria: Alkaptonuria (AKU), also known as black bone disease due to its manifestation of hyperpigmentation, is a rare autosomal recessive disorder caused by the deficiency of homogentisate 1,2-dioxygenase (HGD), which causes an accumulation of homogentisic acid (HGA), some of which is deposited as ochronotic pigment in connective tissues, particularly weight-bearing cartilages [1,2].