<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 13-month-old boy is referred to an immunologist with recurrent otitis media, bacterial sinus infections, and pneumonia, which began several months earlier. He is healthy now, but the recurrent nature of these infections are troubling to his parents and they are hoping to find a definitive cause. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. The patient has five older siblings, but none of them had similar recurrent illnesses. Clinical pathology results suggest very low levels of serum immunoglobulin. As you discuss options for diagnosis with the patients family, which of the following tests should be performed next?? A: Genetic analysis, B: Flow cytometry, C: Urine protein screening, D: Stool cultures, E: CSF gram staining,  <response>: This is a clinical vignette about a 13-month-old boy with recurrent infections suggestive of an underlying immunodeficiency. The key findings are recurrent otitis media, sinusitis, and pneumonia starting a few months ago despite being previously healthy and up-to-date on vaccines. He has low serum immunoglobulin levels. The question is asking which test should be done next to make a definitive diagnosis. Based on the clinical presentation and low immunoglobulins, this patient likely has a primary immunodeficiency disorder affecting B cellsantibody production. Choice A for genetic analysis is incorrect because while genetic testing can help determine the specific immunodeficiency, it is not the next best step diagnostically. Choice B for flow cytometry is the best answer. Flow cytometry allows quantification of different lymphocyte subsets including B cells, T cells, and NK cells. This can help characterize the specific immunodeficiency.Choice C for urine protein screening is unrelated and would not help make a definitive diagnosis. Choice D for stool cultures would not be helpful diagnostically. Choice E for CSF gram staining may be indicated if the patient had symptoms concerning for meningitis but is unrelated to the underlying immunodeficiency.B: Flow cytometry 