You are answering a USMLE-style medical question. Question: An 11-month-old boy is brought to the clinic by his mother for a rash on his trunk, which he has had for the past 2 days. She notes that he is eating less and is more cranky than usual. His birth history is insignificant, and his immunizations are up to date. Vital signs include: temperature is 37.8°C (100.0°F), pulse is 98/min, and respiratory rate is 16/min. The rash features thin-walled, fluid-filled blisters that rupture easily. Fluid samples from the lesions are sent for analysis to a microbiology lab. The results reveal an infection by the gram-positive bacterium Staphylococcus aureus. The patient is diagnosed with staphylococcal scalded skin syndrome. Which of the following is involved with the primary defense in response to the bacteria and toxins produced by this organism? Options: A) Immunoglobulin- IgG B) Immunoglobulin- IgD C) Immunoglobulin- IgA D) Immunoglobulin- IgM Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.