**Core Concept**
Bruton's agammaglobulinemia is a rare genetic disorder characterized by the deficiency of B cells and consequently, the lack of immunoglobulins (antibodies) in the body. This condition is caused by mutations in the BTK gene, which encodes for the tyrosine kinase enzyme responsible for signaling in B cells.

**Why the Correct Answer is Right**
The BTK gene plays a crucial role in the activation of B cells, and its mutation leads to impaired B cell development and function. As a result, Bruton's agammaglobulinemia patients have low levels of immunoglobulins, making them highly susceptible to infections. The BTK enzyme is involved in the signaling pathway initiated by the B cell receptor complex, which is essential for B cell activation, proliferation, and differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a relevant answer, as Bruton's agammaglobulinemia is not caused by a defect in the T cell receptor complex.

**Option B:** While the B cell receptor complex is involved in the signaling pathway, the mutation responsible for Bruton's agammaglobulinemia specifically affects the BTK gene, not the B cell receptor complex itself.

**Option C:** This option is incorrect, as the Bruton's agammaglobulinemia is not caused by a deficiency of the IL-2 receptor, which is involved in T cell activation and proliferation.

**Clinical Pearl / High-Yield Fact**
Bruton's agammaglobulinemia is an X-linked recessive disorder, meaning it primarily affects males, as they have only one X chromosome. Female carriers of the mutation may exhibit mild symptoms due to X-chromosome inactivation.

**Correct Answer: B. Mutation in the BTK gene.**