... consists of a special diet, continued indefinitely.Homocystinuria (<1 in 100,000). Lack of an enzyme that converts the amino acid homo-cysteine into cystathionine, needed for normal brain development. ... after childhood illness. Treatment includes a low-protein diet. Propionic acidemia (>1 in 75,000). Defect in processing four amino acids leading to ill-ness in newborns, including brain damage, ... resulting in lack of the enzyme that recycles the vitamin biotin. May cause frequent infections, uncoordinated movement, hearing loss, seizures, and mental retardation. Undiagnosed and untreated,...