Lily visits her family doctor. He tells her that the upset stomach she experiences after drinking milk is caused by too little lactase, an enzyme that breaks down a sugar in milk into monosaccharides. Which monosaccharides form from this reaction?

Lactose is a disaccharide molecule, which means it is formed by two different simple sugar molecules bonded together; those molecules are glucose and galactose. The body can extract energy from lactose, so it uses an enzyme called lactase to break lactose into two monosaccharides: glucose and galactose.

We can conclude that after lactase breaks down a sugar in milk, the monosaccharides formed in that reaction are glucose and galactose.