A Rare Presentation of A Common Dermatosis; Unilateral Blaschko-Linear Erythema Multiforme.

Erythema multiforme (EM) is a self-limiting, immune-mediated condition characterized by target-like lesions, often triggered by infections, with herpes simplex virus (HSV) being the most common agent. In rare cases, EM may present following Blaschko's lines, a pattern of cutaneous mosaicism. We report a unique case of EM in a 17-year-old male with lesions distributed along Blaschko's lines across his arm, trunk, and leg, marking the most extensive presentation of EM in this pattern to date. The patient had no history of HSV infection or drug use, except for cannabis consumption, which was considered a probable trigger based on the Naranjo Adverse Drug Reaction Probability Scale. Histopathological analysis confirmed the diagnosis of EM, and the lesions resolved with topical clobetasol propionate. This case highlights the rare occurrence of EM along Blaschko's lines in a young patient and suggests cannabis as a potential trigger, contributing to the understanding of atypical presentations and etiology in EM.