American Board of Internal Medicine (ABIM) Certification Practice Exam

Identifying which feature is essential in diagnosing Mixed Connective Tissue Disease?

1. Raynaud phenomenon

2. Sclerodactyly

3. Positive anti-U1-RNP antibodies

4. Intermittent joint pain

The correct answer is: Positive anti-U1-RNP antibodies

The diagnosis of Mixed Connective Tissue Disease (MCTD) is particularly characterized by the presence of positive anti-U1-RNP antibodies. This autoimmune condition presents a unique blend of features commonly found in other connective tissue diseases, such as systemic lupus erythematosus, systemic sclerosis, and polymyositis, but the hallmark serological marker is the presence of these specific antibodies. Anti-U1-RNP antibodies are directed against a component of the ribonucleoprotein complex involved in RNA processing. Their positivity is not only indicative of MCTD but also serves as a distinguishing factor from other connective tissue diseases, making it essential for a definitive diagnosis. Consequently, patients exhibiting these antibodies alongside clinical manifestations typical of MCTD, such as myositis, Raynaud phenomenon, and joint pain, can lead to a comprehensive understanding and identification of the disease. While Raynaud phenomenon and sclerodactyly may present in MCTD, they are not exclusive to this condition and can occur in other autoimmune disorders. Similarly, intermittent joint pain is a common symptom but does not uniquely define MCTD. Thus, the presence of positive anti-U1-RNP antibodies stands out as the critical feature in diagnosing Mixed Connective Tissue Disease.