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
Which of the following tests would be MOST useful for confirmation of Pemphigus vulgaris? 
Select and explain. 
A. Bacterial culture, B. Complete blood count, C. Fungal culture, D. Tissue biopsy with direct immunofluorescence
D: Tissue biopsy with direct immunofluorescence
Answer: D: Tissue biopsy with direct immunofluorescence 
Explanation: Direct immunofluorescence (DIF) is of great value in the early diagnosis of pemphigus vulgaris. DIF shows intercellular IgG throughout the epidermis or the oral epithelium. IgG is found in both involved and clinically normal skin in nearly all patients with pemphigus. In acantholytic areas, C3 deposition is also reliably found. DIF results remain positive for a long time and may still be positive many years after clinical remission. Bacterial culture is helpful in superinfected lesions, but does not help in confirming a diagnosis of pemphigus. Complete blood count is helpful in infectious processes. Fungal culture is used in tinea lesions. Ref: Longo D.L., Fauci A.S., Kasper D.L., Hauser S.L., Jameson J.L., Loscalzo J. (2012). Chapter 54. Immunologically Mediated Skin Diseases. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.