<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 2-year-old boy is brought to the emergency department because of fever, fatigue, and productive cough for 2 days. He had similar symptoms 6 months ago, when he was diagnosed with pneumonia. Three weeks ago, he was diagnosed with otitis media for the sixth time since his birth and was treated with amoxicillin. His temperature is 38.7C 101.7F, pulse is 130min, respirations are 36min, and blood pressure is 8440 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 93. Examination shows purulent discharge in the left ear canal and hypoplastic tonsils without exudate. Coarse crackles are heard over the right lung field on auscultation. An x-ray of the chest shows a right middle lobe consolidation. Flow cytometry shows absent B cells and normal T cells. Which of the following is the most appropriate next step in management?? A: Thymus transplantation, B: Stem cell transplantation, C: Intravenous immunoglobulins, D: Recombinant human granulocyte-colony stimulating factor administration, E: Interferon-γ therapy,  <response>: This is a clinical vignette describing a 2-year-old boy with recurrent infections, including pneumonia, otitis media, and current fever, cough, and hypoxemia concerning for pneumonia again. His history is concerning for an immunodeficiency given the recurrent infections and lack of improvement with antibiotics. The stem provides key information that flow cytometry shows absent B cells and normal T cells, which indicates this patient likely has X-linked agammaglobulinemia XLA, a type of primary immunodeficiency caused by mutations in the BTK gene leading to impaired B cell development and lack of immunoglobulin production. The answer choices list different therapeutic options for immunodeficiencies. Choice A, thymus transplantation, would be indicated for severe combined immunodeficiency SCID to replace defective T cells but not for isolated B cell defects like in this patient with XLA. Choice B, stem cell transplantation, could be curative for XLA by replacing the defective B cells. Choice C, intravenous immunoglobulin IVIG replacement, is the standard of care for XLA to replace the missing antibodies. Choice D, G-CSF, may help treat infections but does not address the underlying immunodeficiency. Choice E, interferon-gamma, may have a role in managing infections in some immunodeficiencies but not specifically indicated in XLA.Based on the clinical presentation of recurrent infections with absent B cells and normal T cells, this patient most likely has XLA. The most appropriate management would be IVIG replacement.C: Intravenous immunoglobulins 