A 52-year-old woman presents with progressive proximal muscle weakness and a violaceous rash over her knuckles and elbows. Examination reveals the lesion marked **B** in the diagram — flat-topped, scaly papules over the extensor surfaces of her metacarpophalangeal and interphalangeal joints. Serum CK is elevated at 3200 U/L. Myositis-specific antibody panel returns positive for anti-TIF-1γ. Which of the following is the MOST important next step in her management?