You are answering a USMLE-style medical question. Question: A 27-year-old woman was referred to a dermatology clinic due to a changing discoloration of her fingers from white to red to blue. Although she has not had any recent changes in her daily routines, she also complains of increasing fatigue, muscle weakness, and weight loss. She has a blood pressure of 126/77 mm Hg, respiratory rate of 14/min, and heart rate of 88/min. Physical examination reveals regular heart and lung sounds. Anti-U1 RNP antibodies and increased creatinine kinase were found in her serum. What is the most likely diagnosis in this patient? Options: A) Mixed connective tissue disease B) Polymyositis C) Systemic sclerosis D) Rheumatoid arthritis Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.