When teaching a mother of a 4-month-old with diarrhea about the importance of preventing dehydration, the nurse would inform the mother that feeding her child with Soy-based, lactose-free formula is advisable. This type of formula is less likely to aggravate diarrhea, since it does not contain lactose. Lactose can cause increased stool output in infants with lactase deficiency, which is common during episodes of diarrhea. Soy-based, lactose-free formulas can help reduce stool output and the duration of diarrhea.