American Board of Internal Medicine (ABIM) Certification Practice Exam

What type of antibodies are associated with Mixed Connective Tissue Disease (MCTD)?

1. Anti-double-stranded DNA antibodies

2. Anti-U1-ribonucleoprotein antibodies

3. Anti-citrullinated protein antibodies

4. Anti-Smith antibodies

The correct answer is: Anti-U1-ribonucleoprotein antibodies

Mixed Connective Tissue Disease (MCTD) is characterized by features of several autoimmune diseases, particularly systemic lupus erythematosus, scleroderma, and polymyositis. A key marker in MCTD is the presence of anti-U1-ribonucleoprotein (RNP) antibodies. These antibodies are directed against components of the small nuclear ribonucleoprotein (snRNP) complex, which plays a crucial role in RNA processing and splicing. The presence of anti-U1-RNP antibodies is significant because it is a strong diagnostic criterion for MCTD, often found in high titers in patients with this condition. They can help differentiate MCTD from other overlapping autoimmune diseases where different autoantibodies might be predominantly present. In contrast, while anti-double-stranded DNA antibodies are primarily associated with systemic lupus erythematosus, anti-citrullinated protein antibodies are typically linked to rheumatoid arthritis, and anti-Smith antibodies are also specific for systemic lupus erythematosus. Therefore, anti-U1-RNP antibodies specifically highlight the unique serological profile associated with MCTD, making them crucial for diagnosis and understanding the nature of this disease.