MEDGENIOover 4 years ago

An 18-month-old boy is brought to the pediatrician for follow-up after his third hospital admission for sepsis due to pneumonia caused by Streptococcus pneumoniae. His mother states that the patient has been having diarrhea today. He was born at 40 weeks without complications, but he has had multiple infections since age 6 months. His mother is worried because her brother died of sepsis at age 5. The patient takes no medications and is otherwise meeting his developmental milestones. Temperature is 37°C (98.6°F), pulse is 120/min, and respiratory rate is 26/min. On examination, he is a thin boy who is small for his age. Tonsils are not visible. Laboratory studies reveal low concentrations of B cells and immunoglobulins but normal T and NK cell counts. A cervical lymph node is biopsied and found to have no germinal centers. Which is the most likely cause of this patient's condition?