MRCP2-2657

A 14-year-old boy is referred to a tertiary immunology service due to recurrent infections since infancy, including sinusitis, otitis media, and pneumonia. He had a prolonged admission to the pediatric intensive care unit with respiratory failure requiring intubation, where he was diagnosed with Pneumocystis jirovecii pneumonia. HIV testing was negative. He also experiences chronic diarrhea, and no cause has been found. Clinical examination revealed stomatitis and gingivitis, but no organomegaly or lymphadenopathy. Blood tests showed low white blood cell count, low lymphocyte count, and low immunoglobulin A levels. What is the likely diagnosis?