It is determined an infant suffers from Bruton's agammaglobulinemia. Which of the following pathogens will present the most serious threat to this child?

Bruton's agammaglobulinemia is a congenital defect that becomes apparent at approximately 6 months of age when maternal IgG is diminished. The child is unable to produce immunoglobulins and develops a series of bacterial infections characterized by recurrences and progression to more serious infections such as septicemia. Cell-mediated immunity is not affected and the child is able to respond normally to diseases that require this immune response for resolution.