A 67 year old man with history of weight loss complains of hoarseness of voice. CT reveals opacity in the right upper mediastinum. He denied any history of difficulty breathing. What is the single most appropriate investigation?

The key is C. Lymph node biopsy. [There is weight loss and there is an opacity in right upper mediastinum. May indicate enlarged lymph node or lymphoma causing pressure on right recurrent laryngeal nerve resulting in horseness. As CT didn't reveal any bronchial lesion and no breathing difficulty it is unlikely to be a bronchial pathology. So CT guided lymph node biopsy can reveal the diagnosis].