Twelve year old boy with vesicle over lip. Investigation to be done is

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