A 6-month-old infant presents with recurrent bacterial infections, absent germinal centers in lymph nodes, and very low levels of circulating B lymphocytes and immunoglobulins, but normal T-cell counts. This presentation is most consistent with a defect in which of the following?
✓ Correct Answer: Option C
X-linked agammaglobulinemia (Bruton's) results from a BTK gene defect, preventing B-cell maturation beyond the pre-B stage. This leads to absent B cells, germinal centers, and immunoglobulins, causing recurrent bacterial infections.