What is the primary differential criterion between erythema multiforme minor and major?

The primary differential criterion between erythema multiforme minor and major is the involvement of mucous membranes. Erythema multiforme minor typically presents with target lesions that are limited to the skin and do not affect the mucous membranes, while erythema multiforme major is characterized by the presence of mucosal involvement, which can lead to complications such as erosions and ulcers in the mouth, eyes, or genital areas.

The distinction between the two forms is important clinically, as the presence of mucosal lesions indicates a more severe form of the disease that may require more intensive management. Other factors, such as systemic symptoms, duration of rash, and severity of itching, may contribute to the overall clinical picture, but they do not serve as the primary differentiating criterion as mucosal involvement does.