SIGNIFICANTLY sub-average intellectual functioning, existing concurrently with related limitations in two or more of the following adaptive skill areas: communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure, and work. Mental retardation manifests before age 18."Sometimes Mental retardation is caused by genetic or metabolic defects that can be inherited from parents, or result from chromosomal abnormalities when genes combine or other genetic reasons. Examples of genetic conditions are Down syndrome, Fragile X syndrome, and phenylketonuria (PKU) Metabolic disorder:- A metabolic disorder occurs when the metabolism process fails and causes the body to have either too much or too little of the essential substances needed to stay healthy.Several disabilities can be caused by recessive genes. One of the most commonly occurring recessive disorders that can cause serious mental retardation is phenylketonuria or PKU.Phenylketonuria is a metabolic disorder.Infants BORN with two recessive genes lack a necessary enzyme that is responsible for converting one of the basic amino acids (phenylalanine) into a product (TYROSINE) that is essential to body functioning.If the enzyme is not present, phenylalanine will continue to build, reaching toxic levels that can damage the central nervous system (CNS).Unchecked, the INFANT will develop PROGRESSIVE mental retardation and if not found before 1 year of age, the infant will become permanently retarded.Detected early, parents of children with PKU are advised to provide the infant a diet low in phenylalanine so that they can develop normally and reach a normal level of intelligence.Hence, we can conclude that Phenylketonuria (PKU), which may lead to mental retardation, is a metabolic disorder.