The Mother of a Client with Cystic Fibrosis (CF)says,"I Don't

The mother of a client with cystic fibrosis (CF) says,"I don't understand why my son is sick so often when some of the other children in our CF support group are hardly ever sick." How should the nurse respond?

A) "We should probably test your son's immune system."
B) "There are many different type of CF,and they affect different people differently."
C) "Are you certain you have been following his medication schedule exactly?"
D) "Children have different levels of disease because they have differing amounts of dysfunction in the gene that causes CF."