A 67-year-old man with lung cancer notices symptoms of weakness in his arms and legs. He describes difficulty in rising out of chair or going upstairs, but no pain or discomfort. On examination, proximal muscle strength is 4 out of 5 in the upper and lower limbs, but the reflexes, tone, and sensation are normal. He has a colored rash on his eyelids, and raised papules on his knuckles.For the above patient with muscle weakness, select the most likely anatomic site for the disorder.