Which of the following tests would be MOST useful for confirmation of Pemphigus vulgaris?

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.