American Board of Internal Medicine (ABIM) Certification Practice Exam

In the diagnosis of mixed connective tissue disease, which autoantibody is commonly found?

1. anti-smooth muscle

2. Anti-U1 RNP (ribonucleoprotein)

3. Anti-Scl-70

4. anti tissue transglutaminase

The correct answer is: Anti-U1 RNP (ribonucleoprotein)

In the context of mixed connective tissue disease (MCTD), the presence of anti-U1 RNP (ribonucleoprotein) antibodies is a hallmark finding. This specific autoantibody is associated with MCTD and plays a significant role in differentiating it from other connective tissue diseases. Patients with MCTD often exhibit signs and symptoms that overlap with those of systemic lupus erythematosus, scleroderma, and polymyositis, making the identification of specific autoantibodies crucial for accurate diagnosis. Anti-U1 RNP antibodies target the ribonucleoprotein complex essential for RNA processing, and their presence is considered a specific marker for MCTD. Other autoantibodies mentioned, such as anti-smooth muscle, anti-Scl-70, and anti-tissue transglutaminase, are more commonly associated with other conditions: anti-smooth muscle antibodies are typically seen in autoimmune hepatitis, anti-Scl-70 antibodies are mostly linked to scleroderma, and anti-tissue transglutaminase antibodies are primarily associated with celiac disease. Therefore, the presence of anti-U1 RNP antibodies is the most indicative of mixed connective tissue disease among the options provided.