Elderly female with rash, limb myasthenia for 1 month, dysphagia, and chest tightness for half a month. Physical examination reveals violaceous eyelid plaques with periorbital edema, coarse breath sounds in both lungs, fine rales/crackles in bilateral lower lungs, tenderness in left hand 2nd and 4th PIP joints, muscle strength grade 3 in upper limbs, and grade 2 in lower limbs. Which of the following is most helpful for definitive diagnosis?