**Core Concept**
Ataxia-telangiectasia is a rare genetic disorder characterized by a combination of immunodeficiency, cancer predisposition, and cerebellar degeneration. The condition results from mutations in the ATM gene, which encodes a protein involved in DNA repair and cell cycle regulation.

**Why the Correct Answer is Right**
The primary antibody deficient in ataxia-telangiectasia is IgA, which plays a crucial role in mucosal immunity. IgA is produced by B cells in mucosal tissues, such as the respiratory, gastrointestinal, and genitourinary tracts, and is responsible for neutralizing pathogens in these areas. The deficiency of IgA in ataxia-telangiectasia leads to increased susceptibility to infections, particularly those affecting the respiratory and gastrointestinal tracts.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most abundant antibody isotype in the blood and plays a key role in systemic immunity. While IgG levels may be affected in ataxia-telangiectasia, the primary antibody deficiency is IgA.

**Option C:** IgM is the first antibody produced in response to an infection and is primarily involved in the early stages of the immune response. IgM levels may be elevated in some cases of ataxia-telangiectasia, but it is not the primary antibody deficient in this condition.

**Option D:** IgD is a minor antibody isotype that is primarily expressed on the surface of mature B cells. IgD is not typically associated with mucosal immunity or deficiencies in ataxia-telangiectasia.

**Clinical Pearl / High-Yield Fact**
Ataxia-telangiectasia is a classic example of a combined immunodeficiency, which means that both T-cell and B-cell functions are impaired. This condition highlights the importance of considering the broader implications of immunodeficiency on patient health and increasing susceptibility to infections.

**✓ Correct Answer: B. IgA**