An 8-month-old boy baby is evaluated because of repeated episodes of pneumococcal pneumoniA Serum studies demonstrate very low levels of IgM, IgG, and IgA This patient's condition is related to a deficiency of which of the following proteins?

As the history is of recurrent infections with bacteria (pneumococcal pneumonia), it indicates primary humoral immunodeficiency. As this is occurring in a 8-month old male child, it indicates X-linked agammaglobulinaemia also called as buon's agammaglobulinaemia. In this disorder there is deficiency of the enzyme tyrosine kinase, which is required for B cell maturation