Alkaptonuria

Alkaptonuria is a rare organic aciduria involving a deficiency in homogentisic acid oxidase, resulting in the accumulation of homogentisic acid (HA), an intermediate in the degradative pathway of tyrosine . The condition has three characteristic symptoms:
homogentisic aciduria (the urine contains elevated levels of HA, which is oxidized to a dark pigment on standing, as shown in Fig. 1A), early onset of arthritis in the large joints, and deposition of black pigment (ochronosis) in cartilage and collagenous tissue (see Fig. 1B). Dark staining of diapers can indicate the disease in infants, but usually no symptoms are present until about age 40 years.

Treatment includes dietary restriction of phenylalanine and tyrosine to reduce HA levels. Although alkaptonuria is not life threatening, the associated arthritis may be severely crippling. [Note: Deficiencies in fumarylacetoacetate hydrolase, the terminal enzyme of tyrosine metabolism, result in tyrosinemia type I  and a characteristic cabbage-like odor to urine.]

Figure 1: Specimens from a patient with alkaptonuria. A. Urine. B. Vertebrae.