What medical condition was identified as the cause of mental retardation in a significant number of institutionalized adults?

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Phenylketonuria (PKU) is a genetic disorder caused by a deficiency in the enzyme phenylalanine hydroxylase. This deficiency leads to an accumulation of phenylalanine in the body, which can cause intellectual disability if not treated early with a special diet. PKU was identified as a significant cause of mental retardation in institutionalized individuals before widespread newborn screening and dietary treatments were implemented.

What Medical Condition Was Identified as the Cause of Mental