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 gestational, B. Bullous pemphigoid, C. Pemphigus vulgaris, D. Herpes simplex", 'role': 'user'}, {'content': 'Answer: C \nExplanation: In the traditional "bulla spread" sign or Lutz sign, the margin of an intact bullae is first marked by a pen. Slow, careful and unidirectional pressure applied by a finger to the bulla causes peripheral extension of the bulla beyond the marked margin. The bulla thus extended has an irregular angulated border in pemphigus vulgaris, while a regular rounded border is observed in bullous pemphigoid or other subepidermal blistering disorders. Ref :Longo D.L., Fauci A.S., Kasper D.L., Hauser S.L., Jameson J.L., Loscalzo J. (2012). Chapter 54. Immunologically Mediated Skin Diseases. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison\'s Principles of Internal Medicine, 18e.', 'role': 'assistant'}]