inborn error of metabolism, one of many abnormal metabolic conditions caused by an inherited defect of a single enzyme or other protein. Although people with such diseases are defective in only one protein, they generally display a large number of physical signs that are characteristic of the genetic trait and are related to excesses or deficiencies of the substrate on which the enzyme acts. The diseases are rare. Kinds include galactosemia, glucose-6-phosphate dehydrogenase (G6PD) deficiency, Lesch-Nyhan syndrome, phenylketonuria, Tay-Sachs disease. ▪ OBSERVATIONS: Inborn errors of metabolism may be detected in the fetus in utero by the examination of squamous and blood cells obtained by amniocentesis and fetoscopy. Laboratory tests after birth often show higher than normal levels of particular metabolites in the blood and urine, such as phenylpyruvic acid and phenylalanine in phenylketonuria (PKU) and galactose in galactosemia. The values are higher in homozygous than in heterozygous carriers. Signs of the various defects are usually seen only in homozygous carriers. ▪ INTERVENTIONS: Treatment for some pathological inborn errors may be removal of food in the diet containing the nondegradable metabolite to prevent its accumulation. Removal of dietary phenylalanine in PKU and galactose in galactosemia is effective in preventing the development of symptoms if treatment is begun early. In those cases of inborn errors of metabolism in which the nondegradable metabolite is endogenous, such as in the mucopolysaccharidoses, no treatment is available.
