A 57-year-old man presents with hemoptysis and generalized weakness. His symptoms began with small-volume hemoptysis 4 weeks ago. Over the past 2 weeks, he has become weak and feels "out of it." His appetite has diminished, and he has lost 10 lb of weight. He has a 45-pack-year history of cigarette smoking. Physical examination is unremarkable. Laboratory studies reveal a mild anemia and a serum sodium value of 118 mEq/L. Chest x-ray shows a 5-cm left, mid-lung field mass with widening of the mediastinum suggesting mediastinal lymphadenopathy. MR scan of the brain is unremarkable. What is the most likely cause of his symptoms?