Which clinical sign is positive in both pemphigus vulgaris and mucous membrane pemphigoid and is elicited by lateral pressure on the mucosa?

A positive Nikolsky sign (sloughing of epithelium with lateral pressure) occurs in vesiculobullous diseases like pemphigus and pemphigoid; perilesional biopsy with direct immunofluorescence confirms the diagnosis.Other options: The Tzanck smear shows acantholytic cells (a test, not this pressure sign); Auspitz sign and Koebner phenomenon relate to psoriasis.Key exam takeaway: Nikolsky sign = epithelial slough with pressure (pemphigus/pemphigoid); confirm with perilesional biopsy and immunofluorescence; pemphigoid can involve the eyes.