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
A 85 year old woman with Nikolsky sign-ve, blisters on thigh & trunk, lesions come on & off. What is the cause : 
Select and explain. 
A. Pemphigus vulgaris, B. Pemphigoid, C. Lichen planus, D. Dermatitis herpetiformis
B: Pemphigoid
Answer: B: Pemphigoid 
Explanation: B i.e. Pemphigoid