An 18-month-old girl is brought to the pediatrician because of recurrent upper respiratory tract infections.She has a fever,labored breathing,and a dry cough.Diagnostic tests confirm infection with Pneumocystis jiroveci.Serum studies are remarkable for negligible levels of IgG and IgA in the presence of high concentrations of IgM.The family history is unremarkable;in particular,the patient's three older brothers are in excellent health.This patient most likely carries a mutation in which of the following molecules?
A)CD18
B)CD40L
C)ZAP-70
D)B7
E)CD40