The most common inborn error of amino acid metabolism that may result in mental retardation is?

• A. Phenylketonuria (PKU)
• B. Maple Syrup Urine Disease
• C. Galactosemia
• D. Alkaptonuria

Answer: (A)

Disruption of phenylalanine metabolism can lead to brain injury and cognitive impairment if not treated, because phenylalanine plus its toxic byproducts accumulates in the brain during development. This happens when phenylalanine hydroxylase is deficient, causing phenylalanine to build up and interfere with normal brain growth. Because of this, the condition is the most common inherited amino acid metabolism disorder associated with mental retardation if untreated, making it the best choice. Early detection through newborn screening allows immediate dietary management to limit phenylalanine intake, often preventing intellectual disability. For contrast, Maple Syrup Urine Disease affects the metabolism of branched-chain amino acids and can cause serious neurological symptoms if not treated, but it is less common and not classically linked to mental retardation in the same way. Galactosemia involves defective galactose metabolism with liver and ocular findings, not primarily a problem of amino acid metabolism. Alkaptonuria is a tyrosine degradation disorder presenting with dark urine and ochronosis, not cognitive impairment from amino acid metabolism.