Which autoantibody is most characteristic of mixed connective tissue disease?

• A. Anti-dsDNA
• B. Anti-U1RNP
• C. Anti-Smith
• D. Anti-SSA

Answer: (B)

The key idea is that mixed connective tissue disease is most closely associated with autoantibodies against U1 small nuclear ribonucleoprotein (U1RNP). These antibodies are strongly linked to the overlapping clinical picture seen in MCTD, which blends features of lupus, scleroderma, and polymyositis. Among the options, anti-U1RNP stands out as the antibody most characteristic of this condition, whereas anti-dsDNA and anti-Smith are classic for systemic lupus erythematosus, and anti-SSA is more typical of Sjögren’s syndrome or SLE but not specifically diagnostic of MCTD. Detecting anti-U1RNP thus provides a meaningful marker when MCTD is suspected. Of course, not every patient with MCTD will have this antibody, and some patients may have overlapping features with other diseases, but it remains the most distinctive marker among the choices.