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?
✓ Correct Answer: Option B
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 caused by a defect in the BTK gene, which is essential for pre-B cell maturation.