A 38-year-old male presents with a 2-year history of low-grade fever and fatigue. Over the past 6 months, he developed myalgia in limb joints and muscles, difficulty climbing 3 floors, and red rashes on eyelids, nasal bridge, and cheeks. He also reports dysphagia with solid foods. Physical exam reveals periorbital edema and violaceous erythema on eyelids, nasal bridge, cheeks, distal interphalangeal joints, and periungual skin. The best diagnostic test is: