Review Question - QID 109601

A 4-year-old male is brought by his mother to the emergency room with dyspnea and fever. His mother reports a two-day history of progressive shortness of breath, malaise, and a fever with a maximum temperature of 101.6°F (38.7°C). The child has visited the emergency room three times over the past two years for pneumonia and otitis media. His family history is notable for sarcoidosis in his mother, diabetes in his father, and an early childhood death in his maternal uncle. His temperature is 101.2°F (38.4°C), blood pressure is 110/90 mmHg, pulse is 110/min, and respirations are 24/min. Physical examination reveals scant lymphoid tissue. A serological analysis reveals decreased levels of IgA, IgG, and IgM. This patient most likely has a defect in a protein that is active in which of the following cellular stages?