Review Question - QID 214899

A 9-month-old boy is brought to the emergency department by his parents because of a fever and difficulty breathing. During the past 3 months, the patient has had several upper respiratory tract infections and poor weight gain. The mother’s pregnancy and delivery were uncomplicated, and the infant’s immunizations are up-to-date. The patient is breastfed, but the mother notes he is eating less lately. He is at the 30th percentile for length and the 5th percentile for weight. His temperature is 101.4°F (38°C), blood pressure is 80/50 mmHg, pulse is 150/min, and respirations are 50/min. Pulse oximetry shows an oxygen saturation of 85% on room air. Physical exam reveals a child in respiratory distress. Crackles are heard diffusely in both lung fields. A chest radiograph demonstrates bilateral, symmetric ground glass opacities. The child is intubated for worsening respiratory distress, and bronchoalveolar lavage (BAL) is performed. Silver staining demonstrates infection with Pneumocystis jirovecii. Laboratory results are notable for the following:

Immunoglobulin A (IgA): 20 mg/dL (normal: 76-390 mg/dL)
Immunoglobulin G (IgG): 100 mg/dL (normal: 650-1500 mg/dL)
Immunoglobulin M (IgM): 500 mg/dL (normal: 40-345 mg/dL)

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