This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': "48 year old male reported with the chief complaint of ulcer on the gums and on general examination headache, fever and malaise was observed. On oral examination, sharply punched out crater-like erosions of the interdental papillae of sudden onset were noted. The patient also complained of bad breath.\n\nThis patient's lower anterior marginal gingiva showed comma-shaped defect called \nSelect and explain. \nA. Stillman's cleft, B. Dehiscence, C. Fenestration, D. Cul-de-sac", 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]