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: Skin
[{'content': "'Bulla spread sign' is seen in: \nSelect and explain. \nA. Herpes gestationalis, B. Bullous pemphigoid, C. Pemphigus vulgaris, D. Herpes simplex", 'role': 'user'}, {'content': 'Answer: C \nExplanation: Pemphigus vulgaris', 'role': 'assistant'}]