Which mutation is responsible for X-linked agammaglobulinemia?

The mutation responsible for X-linked agammaglobulinemia (XLA) is found in the BTK gene, which stands for Bruton's tyrosine kinase. This condition is characterized by a profound deficiency in immunoglobulins due to a failure of B-lymphocyte maturation. The BTK protein plays a critical role in the signaling pathways that are essential for B cell development. Mutations in the BTK gene disrupt this process, leading to the absence of mature B cells and, consequently, low levels of immunoglobulins in affected individuals.

In contrast, the other options pertain to different genetic mutations associated with other disorders. For example, mutations in IL-12R are related to a type of immunodeficiency that affects the immune response to certain infections. CD19 is involved in B cell signaling but is not the mutation responsible for XLA. Similarly, JAK3 mutations are linked to severe combined immunodeficiency (SCID), but they do not cause the specific B cell abnormality seen in XLA. Thus, the unique association of the BTK mutation with the failure of B cell development is what makes it the correct answer in this context.