normally retained in the body because of its high renal clearance; the absence of the HGD enzyme leads to abundant urinary excretion of homogentisic acid, which darkens slowly upon oxidation by prolonged exposure to air. The darkening is hastened by the addition of alkali to the urine and is reflected in the original term for homogentisic acid, alkapton, which refers to its avidity for alkali. ...
