Biochemists generally do not employ the Cahn-Ingold-Prelog system to represent carbohydrate stereochemistry, relying instead on Fischer projection formulas and the D,L system of notation, which relates the stereochemistry of carbohydrate carbons to the configuration of (+)-glyceraldehyde. Although the D,L system finds its way onto almost every MCAT, but not in terms of direct application. You probably would never need to relate a form to (+)-glyceraldehyde on the MCAT and assign the nomenclature yourself. When the D,L system is on the MCAT, it is almost always serving as a comprehension stumbling block. If you understand the basics, you won't have a problem. Of the hexoses (glucose, galactose, fructose, etc.), only the D isomers are naturally occuring.