A patient presents with painful oral erosions that have been present for weeks. Biopsy shows suprabasilar acantholysis. Direct immunofluorescence shows IgG deposits in a 'fishnet' or 'intercellular' pattern throughout the epithelium. What is the diagnosis?

Correct answer: C. Pemphigus vulgaris

• A. Erosive lichen planus shows saw-tooth rete ridges, band-like T-lymphocyte infiltrate, and Civatte bodies on histopathology. DIF shows fibrinogen (fibrin) at the basement membrane — not the intercellular IgG pattern of pemphigus.
• B. Erythema multiforme shows interface dermatitis, necrotic keratinocytes, and perivascular inflammation. DIF is typically negative. It is not characterized by suprabasilar acantholysis or intercellular IgG.
• C. CORRECT. Pemphigus vulgaris (PV) is an autoimmune blistering disease where autoantibodies target DESMOGLEIN (desmosomal proteins). Histopathology shows SUPRABASILAR ACANTHOLYSIS (intraepithelial split, 'row of tombstones' appearance). DIF shows IgG in a fishnet/intercellular pattern throughout the epithelium — pathognomonic for pemphigus.
• D. Mucous membrane pemphigoid (MMP) shows SUBEPITHELIAL cleavage (NOT suprabasilar) on histopathology, and DIF shows linear IgG/IgA at the BASEMENT MEMBRANE ZONE — not the intercellular pattern.