A 58-year-old male smoker presents with a 3-month history of progressive dyspnea, chest pain, and weight loss. Chest X-ray shows a central hilar mass with mediastinal widening. CT chest confirms a 4 cm central lung lesion with ipsilateral hilar and mediastinal lymphadenopathy. Bronchoscopy with endobronchial biopsy is planned. Which investigation is most appropriate to confirm the diagnosis and guide further management?