A 12-year-old boy has had multiple recurrent infections for the past 10 years, including Pneumocystis jiroveci pneumonia, Streptococcus pneumoniae otitis media, and Pseudomonas aeruginosa urinary tract infection. On physical examination, he has a temperature of 38.5deg C and pharyngeal erythema with exudate. Laboratory studies show hemoglobin, 9.1 g/ dL; hematocrit, 27.6%; platelet count, 130,900/mm3; and WBC count, 3440/mm3 with 47% segmented neutrophils, 3% bands, 40% lymphocytes, and 10% monocytes. Serum immunoglobulin levels show very low IgG, very high IgM, and undetectable IgA. A peripheral blood smear shows nucleated RBCs. Which of the following immunologic defects is most likely to produce this disease?