Q. A 3-year-old boy presents to the emergency department with a fever and a history of recurrent bacterial infections. The patient's medical history is significant for recurrent otitis media, which has been treated with multiple courses of antibiotics. He has also experienced recurrent bacterial infections secondary to encapsulated pyogenic bacteria , including pneumonia and sinusitis. He has also experienced recurrent episodes of diarrhea, which have resolved with treatment. The patient's physical examination reveals a young boy who appears pale and thin. His height and weight are below the 5th percentile for his age, consistent with failure to thrive. The patient has no tonsillar tissue, and his nasal passages are congested. There is no evidence of lymphadenopathy or hepatosplenomegaly. Further workup reveals serum levels of all immunoglobulins to be undetectable, with an absent antibody response to vaccinations. The patient's T and B cell counts are normal. Based on the information you have at this point in time, which of the following is the most likely diagnosis?