You are answering a USMLE-style medical question. Question: A 3-month-old boy presents to his pediatrician with persistent diarrhea, oral candidiasis, and signs and symptoms suggestive of respiratory syncytial virus (RSV) pneumonia. His weight is in the 10th percentile. He is being evaluated for an immunodeficiency disease. Laboratory results for the HIV are negative by PCR. Which of the following is the most likely cause of these findings in this patient? Options: A) Defective T cell function B) Grossly reduced levels of B cells C) Defective isotype switching D) Selective IgA deficiency Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.