In a male child presenting with recurrent infections and no mature B cells, what is the most likely diagnosis?

The scenario presented involves a male child with recurrent infections and an absence of mature B cells, which points towards an X-linked genetic defect. In this context, the diagnosis aligns with Bruton's disease, also known as X-linked agammaglobulinemia.

Bruton's disease is characterized by a mutation in the BTK gene, which is essential for the maturation of B cells. As a result of this mutation, affected individuals have minimal to absent mature B cells and consequently lack immunoglobulin production. This deficiency in antibody response leads to the child being highly susceptible to infections, particularly with encapsulated bacteria, as well as certain viral infections.

The other conditions presented do not fully account for the combination of recurrent infections and the specific finding of absent mature B cells. For instance, hyper-IgM syndrome typically involves normal or elevated levels of immunoglobulin M (IgM) but low levels of other immunoglobulin classes due to a defect in class switching, which does not directly relate to the absence of mature B cells. Severe combined immunodeficiency (SCID) can cause recurrent infections, but it typically affects both T and B cells, and it may not specifically show an absence of mature B cells alone. Common variable immunodeficiency