A 14-month-old child has had multiple infections since birth, including pneumonia with Pseudomonas aeruginosa, adenovirus, and Aspergillus fumigatus; diarrhea with Isospora belli; otitis media with Haemophilus influenzae; and urinary tract infection with Candida albicans. Laboratory studies show hemoglobin, 13.2 g/dL; hematocrit, 39.7%; platelet count, 239,100/mm3; and WBC count, 3450/mm3 with 85% segmented neutrophils, 6% bands, 2% lymphocytes, and 7% monocytes. Serum immunoglobulin levels are IgG, 118 mg/dL; IgM, 14 mg/dL; and IgA, 23 mg/dL. The child dies of pneumonia. At autopsy, a hypoplastic thymus, small lymph nodes that lack germinal centers, and scant gut- associated lymphoid tissue are seen. Which of the following is the most likely cause of this disease?

Severe combined immunodeficiency (SCID) can be treated with allogeneic bone marrow transplantation. The transplanted stem cells in the bone marrow give rise to normal T and B cells. Half of SCID cases are caused by an X-linked mutation in the common g chain for cytokine receptors, and the rest are due to autosomal recessive mutations in the gene encoding for adenosine deaminase, which leads to accumulation of metabolites toxic to lymphocytes. An abnormal CD40 ligand interaction with CD40 leads to lack of isotype switching in patients with hyper-IgM syndrome. The BTK gene product is required for differentiation of pro-B cells and pre-B cells, and a mutation leads to agammaglobulinemia. The 22q11 deletion is seen in infants with DiGeorge anomaly and results in lack of T cell development. Individuals lacking complement component C2 have some increase in infections, but mainly develop a disease resembling systemic lupus erythematosus. HIV infection leads to many opportunistic infections, which sometimes occur in infancy and early childhood, but it is mainly CD4+ lymphocytes that are diminished.