A 23 year old woman comes to the clinic because of recurrent sinus infections and 3 episodes of bacterial pneumonia over the past 2 years. She has chronic diarrhea and notes that infections became more frequent during late adolescence. Physical examination is unremarkable. Laboratory studies show low levels of IgG, IgA, and IgM with a normal number of circulating B lymphocytes. Which of the following is the most likely underlying defect?

1. A)Failure of B cells to differentiate into plasma cellsGABARITO
2. B)Defect in CD40 ligand on T helper cells
3. C)Deficiency of the NADPH oxidase complex
4. D)Mutation in the BTK tyrosine kinase gene
5. E)Absence of thymic development from the third and fourth pharyngeal pouches