This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
It is determined an infant suffers from Bruton's agammaglobulinemia. Which of the following pathogens will present the most serious threat to this child? 
Select and explain. 
A. Measles virus, B. Mycobacterium tuberculosis, C. Chlamydia trachomatis, D. Varicella-zoster virus (VZV)
C: Chlamydia trachomatis
Answer: C: Chlamydia trachomatis 
Explanation: 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.