A 67-year-old man who is a long-term smoker presents with weight loss, a persistent cough, fever, chest pain, and hemoptysis. A chest x-ray reveals a 3.5cm hilar mass on the right and postobstructive pneumonia of the right lower lobe. Sputum cytologic is suspicious for malignant cells. Histologic examination of a transbronchial biopsy specimen reveals in infiltrating groups of cells with scant cytoplasm. No glandular structures or keratin production are seen. The nuclei of these cells are about twice the size of normal lymphocytes and do not appear to have nucleoli. Which of the following is the most likely diagnosis?