American Board of Internal Medicine (ABIM) Certification Practice Exam

The presence of which antibodies is a hallmark in the diagnosis of Mixed Connective Tissue Disease?

1. Antinuclear antibodies

2. Anti-histone antibodies

3. Anti-U1-ribonucleoprotein antibodies

4. Anti-Ro/SS-A antibodies

The correct answer is: Anti-U1-ribonucleoprotein antibodies

The presence of anti-U1-ribonucleoprotein antibodies is indeed a hallmark in the diagnosis of Mixed Connective Tissue Disease (MCTD). These antibodies are considered a specific serological marker for MCTD and indicate the presence of an autoimmune process involving multiple connective tissues. MCTD is characterized by features of systemic lupus erythematosus, scleroderma, and polymyositis, and the detection of anti-U1-ribonucleoprotein antibodies helps to differentiate it from these other conditions. The presence of these antibodies suggests a unique immunological profile that is critical for confirming the diagnosis of MCTD. In contrast, while antinuclear antibodies may be present in various autoimmune diseases, including MCTD, they are not specific to it. Anti-histone antibodies are more closely associated with drug-induced lupus and less directly related to MCTD. Anti-Ro/SS-A antibodies, although commonly associated with Sjögren's syndrome and systemic lupus erythematosus, do not serve as a definitive marker for MCTD either. The specificity of anti-U1-ribonucleoprotein antibodies for MCTD makes them crucial for accurate diagnosis in this context.