Alkaptonuria: A Case Report With Diagnostic Challenge

Alkaptonuria is a rare autosomal recessive metabolic disorder caused by deficiency of homogentisic acid (HGA) oxidase, the only enzyme capable of catabolizing HGA. Deficiency of this enzyme leads to excess HGA which deposits in the connective tissue. We present a case of a 64-year-old woman who was referred to the dermatology clinic for a full body mole check and skin cancer screening. Clinically she had blue/gray pigmentation of the external ear and sclera. Also she had a domed papule on the left cheek with punctate gray pigmentation which was biopsied. Histopathological examination showed a benign dermal nevus and nonpolarizable, yellow-brown, irregular shaped fibers. Subsequent organic acid screen showed markedly elevated urinary HGA, diagnostic of alkaptonuria. On specific inquiry, the patient revealed she had a history of bilateral Achilles tendon rupture, black urine, arthritis, and external ear discoloration for many years. The pigmented material was then considered to be HGA deposition within the dermal collagen fibers. However, without the appropriate clinical data and confirmatory lab findings, the pigmented fragments on skin biopsy represent a diagnostic challenge. Measures like low protein diet and ascorbic acid supplementation will slow down the disease progression and potential complications later in life; however, there is no definitive treatment for the disease. We emphasize the prompt recognition of the clinical signs and symptoms as well as the importance of the microscopic findings.