A 2-year-old boy is brought to the clinic for evaluation of recurrent sinus and pulmonary infections. He has no history of opportunistic infections. Physical examination reveals no lymphadenopathy or tonsillar enlargement. Laboratory studies show normal B-cell and T-cell counts. Quantitative serum immunoglobulins reveal markedly elevated IgM levels with very low IgG, IgA, and IgE levels. His maternal uncle died in childhood from severe infections. Which of the following molecular defects most likely underlies this patient’s immunologic findings?