Classic galactosemia (CG) is an inborn error of galactose metabolism caused by a severe deficiency of galactose-1-phosphate:uridylyltransferase (GALT) enzymatic activity, which leads to accumulation of several metabolites such as galactose-1-phosphate (Gal-1-P), the substrate of GALT, and galactonate and galactitol, the oxidation and reduction products.