A 7-month-old boy presents with recurrent bacterial infections including otitis media and pneumonia caused by *Streptococcus pneumoniae*. Laboratory studies show profoundly reduced levels of all immunoglobulin classes (IgG, IgM, IgA) and absent tonsils on examination. Flow cytometry reveals severely reduced B-cell numbers with arrest at the pre-B-cell stage in the bone marrow. The structure marked **C** in the diagram represents the genetic defect responsible for this clinical presentation. Which of the following best describes the molecular basis of the condition marked **C**?

Explanation

## Why "Mutation in the BTK gene on the X chromosome leading to arrest of B-cell development at the pre-B-cell stage" is right Bruton X-linked agammaglobulinemia (XLA), marked as **C**, is caused by mutations in the BTK (Bruton tyrosine kinase) gene located on the X chromosome. This gene encodes a non-receptor tyrosine kinase essential for B-cell receptor signaling and B-cell development. Loss of BTK function results in arrest of B-cell maturation at the pre-B-cell stage in the bone marrow, preventing the development and emigration of mature B-cells to the periphery. This explains the profoundly reduced immunoglobulin levels (all classes), absent lymphoid tissue (tonsils, germinal centers), and the clinical presentation of recurrent bacterial infections with encapsulated organisms after maternal IgG wanes (typically after 6 months of age). T-cell function remains intact, explaining why the patient can clear most viral and intracellular infections. (Robbins 10e Ch 5; Harrison 21e Ch 351) ## Why each distractor is wrong - **Deletion of chromosome 22q11 resulting in thymic aplasia and absent T-cell development**: This describes DiGeorge Syndrome (marked as **D** in the diagram), not **C**. DiGeorge presents with absent T-cells, normal B-cell numbers, and recurrent viral/intracellular infections rather than bacterial infections. Immunoglobulin levels are typically normal or elevated. - **Mutation in the RAG1/RAG2 genes causing failure of V(D)J recombination in both T and B cells**: This describes Adenosine Deaminase (ADA) deficiency or Recombination Activating Gene (RAG) deficiency, which causes Severe Combined Immunodeficiency (SCID). SCID presents with absent both T and B cells, whereas XLA has intact T-cells and specifically absent B-cells. - **Mutation in the FOXP3 gene leading to loss of regulatory T cells and autoimmune manifestations**: FOXP3 mutations cause IPEX syndrome (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked), characterized by autoimmunity and diarrhea, not the recurrent bacterial infections seen in XLA. B and T cell numbers are present but dysregulated. **High-Yield:** XLA = BTK mutation → no B-cells → no immunoglobulins → recurrent bacterial infections (especially encapsulated organisms) after 6 months; T-cells intact; avoid live vaccines. [cite: Robbins 10e Ch 5; Harrison 21e Ch 351]