A 7-year-old boy presents to the immunology clinic with a history of recurrent sinopulmonary infections and persistent ear infections since early childhood. His laboratory evaluation reveals markedly elevated serum IgM levels with very low IgG, IgA, and IgE levels. Lymphocyte counts are within normal limits. These findings suggest a defect in B-cell isotype switching. Which enzyme deficiency is most likely responsible for his immunodeficiency? Laboratory Data: • IgM: Elevated • IgG, IgA, IgE: Markedly decreased • Total lymphocyte count: Normal Note: The defect is intrinsic to the B cells and involves the inability to initiate class switch recombination.