The caregivers of an 8-year-old bring their child to the pediatrician and report that the child has not had breathing problems before, but, since taking up lacrosse, the child has been coughing and wheezing at the end of every practice and game. Their friend's child has often been hospitalized for asthma; they are concerned that their child has a similar illness. The nurse knows that because the problems seem to be directly related to exercise, it is likely that the child will be able to be treated with what?