A 40 year old male reported with recurrent episodes of oral ulcers, large areas of denuded skin and flacid vesiculo-bullous eruptions.Which is the most important bed-side investigation helpful in establishing the diagnosis -

• A. Skin biopsy and immunoflurescence
• B. Tzanck smear from the floor of bulla
• C. Gram staining of the blister fluid
• D. Culture and sensitivty