Developmental delay may reflect chronic energy deficit in the brain (eg, oxidative phosphorylation defects), decreased supply of needed carbohydrates that are non-energy substrates for the brain (eg, lack of uridine-5 ′ - diphosphate-galactose [UDP-galactose] in untreated galactosemia), or chronic amino acid deficit in the brain (eg, tyrosine deficiency in phenylketonuria).