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 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 - 
Select and explain. 
A. Gram staining of the blister fluid, B. Culture and sensitivity, C. Skin biopsy and immunoflurescence, D. Tzanck smear from the floor of bulla
D: Tzanck smear from the floor of bulla
Answer: D: Tzanck smear from the floor of bulla 
Explanation: D. i.e. Tzank smear