A 6-month-old male infant presents with recurrent bacterial infections, including otitis media and pneumonia, since 3 months of age. Laboratory findings show significantly decreased serum immunoglobulin levels (IgG, IgA, IgM) and an absence of circulating B lymphocytes, while T cell counts and function are normal. Which of the following is the most likely underlying genetic defect in this patient?

The correct answer to the question "A 6-month-old male infant presents with recurrent bacterial infections, including otitis media and pneumonia, since 3 months of age. Laboratory findings show significantly decreased serum immunoglobulin levels (IgG, IgA, IgM) and an absence of circulating B lymphocytes, while T cell counts and function are normal. Which of the following is the most likely underlying genetic defect in this patient?" is Option B: Bruton's tyrosine kinase (BTK) gene mutation.

The other options — A) Adenosine deaminase deficiency, C) CD40 ligand deficiency, D) Myeloperoxidase deficiency — are incorrect for this USMLE Exam Prep question. As explained above: The clinical picture of recurrent bacterial infections, absent B cells, and low immunoglobulins, with normal T cell function, is classic for X-linked agammaglobulinemia (Bruton's). This condition is …