Which of the following most logically completes the argument below?
Using antibiotics to kill Toxoplasma, a parasite that attacks animals with a weak immune system, presents a difficulty: how to keep the immune system from being damaged along with the parasite. For at least some species of animals, a special medicine that prevents antibiotic-based damage of the immune system, is being developed. This protection wears off as the animal matures. Therefore, the special medicine will be especially useful for Toxoplasma-vulnerable animals that _____________ .