inherited inability of the body to metabolize normally the amino acid tyrosine. In the normal metabolic pathway of tyrosine, para-(p-)hydroxyphenylpyruvic acid is converted to homogentisic acid (in the liver) by a specific organic catalyst or enzyme, called p-hydroxyphenylpyruvic acid oxidase. This enzyme is not active in individuals with tyrosinemia. Clinical features of the disease include an increase in concentration of tyrosine in the blood and urine, defects in kidney function, and replacement of liver cells with fibrous tissue. There may also be fewer circulating platelets in the blood than normal and an increased pigmentation of the skin. Affected children may develop a form of rickets that does not improve with administration of vitamin D. A diet low in tyrosine and phenylalanine, an amino acid that the body converts to tyrosine, helps keep tyrosinemia under control. This disorder is extremely rare; it is transmitted through a recessive gene.