<prompt> A 40 year old male repoed with recurrent episodes of oral ulcers, large areas of denuded skin and flacid vesiculo-bullous eruptions. Which is the most impoant bed-side investigation helpful in establishing the diagnosis -Gram staining of the blister fluidCulture and sensitivitySkin biopsy and immunoflurescenceTzanck smear from the floor of bulla  <response>: D. i.e. Tzank smear 