What clinical sign is used to diagnose pemphigus vulgaris?

Nikolsky's sign is the hallmark clinical finding in pemphigus vulgaris. It is elicited by applying gentle lateral pressure to the perilesional skin or mucosa — this causes the epidermis to separate and slide off, forming a new blister. This occurs because autoantibodies (IgG) target desmogleins (desmoglein 3 and 1), the proteins that hold keratinocytes together via desmosomes, causing acantholysis (loss of cohesion between epidermal cells). Nikolsky's sign is positive in pemphigus vulgaris but negative in bullous pemphigoid. Definitive diagnosis requires biopsy with histology (showing intraepithelial acantholysis) and direct immunofluorescence.