Which condition is defined as having less than 10% body surface area affected?

The condition characterized by having less than 10% of the body surface area affected is Erythema Multiforme. This condition is known for its target-shaped lesions and is typically associated with infections (like herpes simplex) or certain medications.

In contrast, Stevens-Johnson syndrome is a severe reaction involving more than 10% of the body surface area, often resulting in extensive epidermal necrosis and significant morbidity. Toxic Epidermal Necrolysis is an even more severe form, affecting over 30% of the body surface area, and has a much higher mortality rate. Psoriasis, while it can vary in extent, is a chronic inflammatory skin condition that can affect large areas but is not categorized solely based on the percentage of body surface area involved.

Therefore, Erythema Multiforme is the correct condition defined specifically by the involvement of less than 10% of the body surface area.