A 6-month-old male infant presents with recurrent severe bacterial pneumonia, otitis media, and skin abscesses. Laboratory evaluation reveals normal numbers of circulating T lymphocytes, but serum IgG, IgA, and IgM levels are virtually undetectable, and CD19/CD20 B cells are absent in peripheral blood. What is the diagnosis?

Answer and explanation

Correct answer: C. X-linked agammaglobulinemia (Bruton's agammaglobulinemia)

  1. Step 1: Analyze immunological profile: Profound pan-hypogammaglobulinemia with complete absence of circulating mature B lymphocytes in an infant whose maternal IgG antibodies have waned (at 6 months of age).
  2. Step 2: Identify molecular etiology: Defect in Bruton tyrosine kinase (BTK) gene on the X chromosome, halting B-cell maturation at the pre-B stage.
  3. Step 3: Conclude diagnosis: X-linked agammaglobulinemia.
Answer
X-linked agammaglobulinemia (Bruton's agammaglobulinemia).
Why not the others: Normal T cells exclude SCID and DiGeorge syndrome; common variable immunodeficiency presents later with preserved B cell numbers; selective IgA deficiency spares IgG and IgM.

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