A newborn is diagnosed with severe recurrent infections. Lab tests show very low levels of IgA, IgG, and IgE, but high levels of IgM. Which of the following defects is most likely?

• A. Deficiency in CD40 ligand
• B. Deficiency in Bruton’s tyrosine kinase (BTK)
• C. Deficiency in complement C3
• D. Deficiency in IL-2 receptor

Answer: (A)