Not a characteristic autoantibody for mixed connective tissue disease?

• A. Anti-U1RNP
• B. Anti-U1RNP, anti-SSA
• C. Anti-ds-DNA
• D. Anti-U1-RNP, anti-phospholipid antibody

Answer: (C)

Autoimmune overlap in mixed connective tissue disease centers on antibodies against U1-RNP. This antibody is a defining feature and explains the blend of lupus-like, scleroderma-like, and myositis symptoms seen in MCTD. Other antibodies can appear variably, such as anti-SSA or anti-phospholipid antibodies, but the hallmark is anti-U1RNP.

Anti-dsDNA, on the other hand, is highly specific for systemic lupus erythematosus and is strongly associated with lupus nephritis. It is not characteristic of mixed connective tissue disease, which is why it’s the best choice for not fitting the typical autoantibody pattern of MCTD.