Q. A 62-year-old male with a 40-pack-year smoking history presents to the clinic with a persistent cough, shortness of breath, and fatigue. His vital signs are as follows: temperature 98.6°F (37°C), blood pressure 135/85 mmHg, heart rate 92 bpm, and respiratory rate 18 breaths/min. A chest CT scan reveals a centrally located perihilar mass. Biopsy of the mass shows small, spindle-shaped cells with a high mitotic rate and scant cytoplasm. Which of the following biomarkers would be most helpful in confirming the diagnosis of this patient's lung cancer?