A 20 year old girl presented with Raynaud's phenomenon, ahralgia, acrosclerosis, and mild myositis. She is positive for a distinctive antibody against U1-ribonucleoprotein (RNP). This antibody is a common finding in which of the following conditions?

Mixed connective-tissue disease (MCTD) is an overlapping of clinical features of systemic lupus erythematosus (SLE), scleroderma, and myositis.MCTD is associated with the presence of high titers of anti-U1RNP antibodies specifically directed against the proteins A, C, or 70K complexed with U1RNA. Anti-RNP bodies are, however, are not specific for MCTD as they are also seen in other connective tissue disorders. Absence of U1 RNP antibodies essentially rules out mixed connective tissue disorder. Ref: Diagnostic Criteria in Autoimmune Diseases edited by Yehuda Shoenfeld, Ricard Cervera, M. Eric Gershwin, 2008, Page 45 ; Harrison 17thedition, Table 325-4