Inability to metabolize galactose occurs in the galactosemias, which may be caused by inherited defects of galactokinase, uridyl transferase, or 4-epimerase, though deficiency of uridyl transferase is best known. Galactose is a substrate for aldose reductase, forming galactitol, which accumulates in the lens of the eye, causing cataract. Ultimately, liver failure and mental deterioration result. Ref: Bender D.A., Mayes P.A. (2011). Chapter 21. The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.