Review Question - QID 213796

A 6-year-old boy is brought to his pediatrician for fever and cough. His mother states that the cough started yesterday and this morning he woke up drenched in sweat. The patient has a history of treatment-resistant bilateral otitis media. He's been hospitalized 5 times for pneumonia. His first hospitalization was at 8 months old. He takes no medications. He has an older brother and an older sister who are both healthy. There is no family history of immunodeficiencies. The patient's entire family is vegetarian, including himself. He does well in school but gets tired easily. The patient's temperature is 101.5°F (38.6°C), blood pressure is 100/54 mmHg, pulse is 98/min, and respirations are 18/min. Bilateral diminished breath sounds with rales are appreciated on lung auscultation. There are no palpable lymph nodes. A chest radiograph shows bilateral consolidations and a thymic shadow. Laboratory tests are obtained as shown below:

Hemoglobin: 13.9 g/dL
Leukocyte count: 3,850 cells/mm^3
Neutrophils: 88%
Lymphocytes: 12%
Platelet count: 350,000/mm^3

Serum:
IgG: 145 mg/dL (Normal: 340-1000 mg/dL)
IgA: 26 mg/dL (Normal: 30-230 mg/dL)
IgM: 29 mg/dL (Normal: 40-200 mg/dL)

Which of the following is the most likely cause of the patient’s presentation?