A 10-Month-Old Boy Is Hospitalized for Respiratory Distress Due to Pneumocystis

A 10-month-old boy is hospitalized for respiratory distress due to Pneumocystis jiroveci pneumonia.  The patient has had repeated episodes of otitis media and thrush; weight is below the 5th percentile.  Laboratory evaluation reveals normal levels of circulating B and T cells with low levels of all immunoglobulins.  Genetic testing reveals a rare autosomal recessive mutation resulting in a defect in the regulation of human leukocyte antigen gene transcription.  Flow cytometry of the patient's peripheral blood compared with a healthy control is shown in the illustration below: Which of the following processes is most likely to be impaired by this patient's mutation?

A) Development of pharyngeal pouches
B) Capability of activated CD4⁺ T cells to express CD40L
C) Maturation of pro-B cells into pre-B cells
D) Presentation of antigens processed in lysosomes
E) Transport of cytosolic proteins into the endoplasmic reticulum

Correct Answer:

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