Name a primary immunodeficiency characterized by absent B cells and agammaglobulinemia in males due to BTK mutation.

• A. X-linked agammaglobulinemia (Bruton’s agammaglobulinemia)
• B. Severe combined immunodeficiency
• C. Common variable immunodeficiency
• D. DiGeorge syndrome

Answer: (A)

This question hinges on recognizing a mutation that specifically halts B cell development, leading to absent B cells and very low immunoglobulins in males. Bruton’s tyrosine kinase (BTK) is essential for progression of B cells from early precursors to mature, antibody-producing cells. A BTK mutation blocks this maturation, so there are essentially no mature B cells in the blood and immunoglobulin levels (IgG, IgA, IgM) are greatly reduced or absent. Because the defect is X-linked, it predominantly affects males and typically presents after maternal antibodies wane around 6 months, with recurrent bacterial infections due to humoral deficiency. The name for this condition is X-linked agammaglobulinemia (Bruton’s agammaglobulinemia). Other primary immunodeficiencies involve different arms of the immune system or have different timing and inheritance patterns: severe combined immunodeficiency involves both T and B cell defects; common variable immunodeficiency presents later with hypogammaglobulinemia but usually with some B cells; DiGeorge syndrome centers on T cell deficiency from thymic hypoplasia.