This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Lung Tumor, Subject name: Pathology
[{'content': 'A 60-year-old male, chronic smoker, presented with fatigue and chest pain. On examination, there was inset left eyeball with drooping upper left eyelid and reduced left pupil size. Which of the following is the most likely diagnosis in this condition? \nSelect and explain. \nA. Squamous cell carcinoma involving the apex of his left lung, B. Adenocarcinoma involving left mainstem bronchus, C. A small-cell carcinoma involving the hilum of his left lung, D. Bronchioloalveolar carcinoma involving the left lower lobe', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Most tumors of the apex of the lung are adenocarcinomas Pancoast tumors - apical lung tumors in the superior pulmonary sulcus tend to invade the neural structures around the trachea, including the cervical sympathetic plexus. Involvement of sympathetic fibers gives rise to severe pain in the distribution of the ulnar nerve and Horner syndrome. Features of Horner syndrome are: S Sympathetic Injury A Anhidrosis M Miosis P Ptosis L Loss of ciliospinal reflex E Enophthalmos', 'role': 'assistant'}]