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
'Bulla spread sign' is seen in: 
Select and explain. 
A. Herpes gestationalis, B. Bullous pemphigoid, C. Pemphigus vulgaris, D. Herpes simplex
C: Pemphigus vulgaris
Answer: C: Pemphigus vulgaris 
Explanation: Pemphigus vulgaris