A 63-year-old man presents with weakness and hemoptysis, but no fever, cough, or sputum. He has a 60-pack-per-year history of smoking. The chest x-ray (CXR) reveals a lung mass with mediastinal widening. On examination, there is a blue purple discoloration of the upper eyelids and erythema on his knuckles. He has proximal muscle weakness rated 4+/5, normal reflexes, and sensation. Which of the following is the most likely diagnosis for his muscle weakness?