<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 3-month-old boy presents to his pediatrician with persistent diarrhea, oral candidiasis, and signs and symptoms of respiratory syncytial virus RSV pneumonia. He is very lean with weight in the 10th percentile. His blood pressure is 10564 mm Hg and heart rate is 84min. He is being evaluated for an immunodeficiency. Laboratory results for HIV are negative by polymerase chain reaction PCR. Which of the following is the most likely cause of this childs presentation?? A: An X-linked inheritance of HLA genes, B: Defective T cell function, C: Selective IgA deficiency, D: Defective isotype switching, E: Grossly reduced levels of B cells,  <response>: This is a medical question asking about the likely cause of a 3-month-old boys presentation with diarrhea, oral candidiasis, and RSV pneumonia, along with being underweight and undergoing evaluation for immunodeficiency. The choices involve various types of immunodeficiencies.Based on the symptoms and age, this sounds like it could be a primary immunodeficiency affecting T cells, as they often present early in infancy. Oral candidiasis and susceptibility to infections points towards T cell dysfunction. The negative HIV PCR makes acquired immunodeficiency less likely. B: Defective T cell function 