Galactosemia is an inherited autosomal recessive genetic disease characterized by failure-to-thrive in infancy, cataracts, and mental retardation. These symptoms are caused by a deficiency of the epimerase enzyme (galactose-1-phosphate uridyl transferase) that normally converts galactose to glucose. The resulting buildup of galactose in the bloodstream causes organ and tissue damage and can potentially cause fatal brain damage.
The treatment for galactosemia is to remove foods that contain galactose from the patient's diet. Any foods containing lactose will by default also contain galactose and must be avoided. Dairy products are an obvious candidate for dietary removal, but other foods like garbanzo beans and tomatoes have too much galactose to be safe. Even if problem foods are avoided, some afflicted children suffer from learning disabilities, and girls can develop ovarian problems. However, many people can lead full, normal lives with this disorder.
Some of the information in this writeup was gleaned from http://depts.washington.edu/transmet/gal.html. The rest is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/