A 4-month-old boy is diagnosed with a life-threatening Pseudomonas infection. What underlying condition might he have?

The diagnosis of Bruton's X-linked agammaglobulinemia is a strong consideration in this clinical scenario. This condition, caused by a mutation in the BTK (Bruton tyrosine kinase) gene, leads to a failure in the development of B cells, resulting in significantly reduced levels of immunoglobulins in the bloodstream. Consequently, patients with this disorder are highly susceptible to extracellular bacterial infections, such as those caused by Pseudomonas aeruginosa, because they lack the antibodies needed to effectively combat such pathogens.

At 4 months of age, the absence of maternal immunoglobulins can leave the child particularly vulnerable to infections, since he is not yet able to produce his own antibodies due to the underlying B cell deficiency. Children with Bruton's agammaglobulinemia often present with recurrent bacterial infections early in life, and due to the persistence of encapsulated organisms, they are at risk for severe infections from certain bacteria like Pseudomonas.

While other conditions listed also involve immunodeficiencies, they typically present with a different type of susceptibility or clinical picture. For instance, Wiskott-Aldrich syndrome includes not only infections but also thrombocytopenia and eczema, while Hyper-Ig