<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 7-month-old Caucasian male presents with recurrent sinusitis and pharyngitis. The parents say that the child has had these symptoms multiple times in the past couple of months and a throat swab sample reveals the presence of Streptoccocus pneumoniae. Upon workup for immunodeficiency it is noted that serum levels of immunoglobulins are extremely low but T-cell levels are normal. Which of the following molecules is present on the cells that this patient lacks?? A: CD4, B: CD8, C: CD3, D: CD19, E: NKG2D,  <response>: This is a question about immunodeficiency in a 7-month-old male with recurrent sinusitis and pharyngitis. The key details are low immunoglobulin levels but normal T-cell levels. This indicates a defect in B cellsimmunoglobulin production rather than T cells. Of the choices given, CD19 is a marker found on B cells. The other options are markers for T cells CD4, CD8, CD3 or NK cells NKG2D. Therefore, the lack of CD19 B cells is most consistent with the clinical picture described.D: CD19 