**Question:** An 8-month-old boy with a history of recurrent pneumonia is found to have almost no circulating IgG. Cellular immunity is normal. His brother had this same disease and died of echovirus encephalitis. His parents and sisters have normal serum levels of IgG. What is the appropriate diagnosis?

A. Common variable immunodeficiency (CVID)
B. X-linked agammaglobulinemia
C. DiGeorge syndrome
D. Severe combined immunodeficiency (SCID)

**Correct Answer:** B. X-linked agammaglobulinemia

**Core Concept:**
Agammaglobulinemia refers to a group of immunodeficiency disorders characterized by a deficiency of circulating immunoglobulins (proteins that play a crucial role in immune response). In the case of X-linked agammaglobulinemia (XLA), the deficiency is caused by mutations in the Bruton's tyrosine kinase (BTK) gene, which is essential for B-cell development and function. Since B cells are responsible for producing immunoglobulins, this results in a profound deficiency of IgG, IgA, and IgM, leaving the patient susceptible to recurrent infections, especially respiratory tract infections.

**Why the Correct Answer is Right:**
The correct answer is X-linked agammaglobulinemia (XLA) because:

1. Circulating IgG is almost absent in the patient, consistent with the disease phenotype.
2. CVID, DiGeorge syndrome, and SCID are different conditions with distinct clinical features and genetic causes.

**Why Each Wrong Option is Incorrect:**
A. CVID (Common variable immunodeficiency):
- CVID is characterized by a broad spectrum of immunoglobulin deficiencies, not just IgG deficiency.
- The patient's clinical presentation and family history differ from CVID, as there are no other affected family members in the question.

C. DiGeorge syndrome:
- DiGeorge syndrome is a chromosomal abnormality with clinical features unrelated to the presented symptoms (no history of pneumonia in family members).

D. SCID (Severe combined immunodeficiency):
- SCID is characterized by a complete absence of T-cells and B-cells, leading to profound immunodeficiency and severe infections.
- The patient's IgG deficiency and normal cellular immunity in the family members rule out SCID.

**Clinical Pearl:**
A clinical pearl in this question is the importance of considering familial immunodeficiency diseases in differential diagnosis when evaluating a child with recurrent infections and absent or severely reduced immunoglobulin levels, particularly when there is a family history of similar cases. This can help narrow down the differential diagnosis and guide targeted testing and management.