This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Photosensitive disorders, Subject name: Dental
Twelve year old boy with vesicle over lip. Investigation to be done is 
Select and explain. 
A. Tzank smear, B. Woods lamp, C. Diascopy, D. KOH mount
A: Tzank smear
Answer: A: Tzank smear 
Explanation: Tzanck smear is a bed side test made by scrapping either floor of bulla( after deroofing). Or floor of erosion ( after removing crust). Or mucosal ( usually oral ) erosion . Stained with Giemsa stain. Acantholytic cells - rounded keratinocytes with relatively large nucleus and a condensed cytoplasm- seen in pemphigus group of disorders-pemphigus vulgaris and pemphigus foliaceous balloning degeneration and multinucleated gaint cells - seen in herpes simplex, herpes zoster and varicella iadvl textbook of dermatology, page 110