The correct answer is E. anti-Sm.
Anti-Sm is diagnostic of SLE and not reported in patients with other connective tissue diseases.
Anti-dsDNA antibodies are typically seen in lupus however they have been reported in MCTD and Sjogrens. It is found in 15-40% of patients with SLE. Most patients with anti-Sm also have antibodies to U1RNP, but the converse is not true. anti-U1RNP is found in 100% of patients with MCTD and in 30% of patients with SLE (the majority of patients with positive U1RNP have SLE rather than MCTD). Anti-dsDNA correlates with renal involvement in SLE. Anti-Ro and anti-La antibodies are found in LE and Sjogren’s, and strongly associated with photosensitivity.
References: PMID: 28281463