**2. Concepts, terms, and classification**

In the SCARs group, in addition to SJS/TEN, there are other forms of drug allergy such as drug reaction with eosinophilia and systemic symptoms (DRESS syndrome), acute generalized exanthematous pustulosis (AGEP), drug-induced hypersensitivity syndrome (DIHS), maculopapular exanthema (MPE).

Previously, the classification between erythema multiforme (EM), SJS, and TEN was still inconsistent due to unclear pathogenesis. There are different opinions on the differential diagnosis of severe EM from SJS/TEN. Since 1983, SJS has been considered synonymous with severe EM, both with the involvement of more than one mucosa with skin involvement [1]. In 1993–1994, Bastuji-Garin and Roujeau proposed the distinction of the two diseases based on clinical and etiological factors. In severe EM, there are mucosal lesions, bullae, skin lesions less than 10% of body surface area [2]. But unlike SJS, the skin lesions in severe EM are typical and/or atypical target lesions that are prominent compared to normal skin, distributed mainly in the extremities. Stevens-Johnson syndrome is characterized by widespread blisters due to drug reactions, which appear on the background of erythema, necrosis, and pruritus, concentrated mainly on the face and trunk. Etiologically, EM is often associated with herpes simplex virus reactivation, rarely drug-induced, SJS/TEN mainly drug-induced, rarely infection [3, 4]. There are few reports of stomatitis caused by *Mycoplasma pneumonia*, mostly in young people, characterized by primary mucosal lesions, with little or no skin lesions. This form is called *Mycoplasma pneumonia*related mucositis [5]. Today, EM is considered as a separate disease, separate from the SJS/TEN group, with specific clinical, epidemiological, and pathophysiological features. Due to the similarity in clinical and histopathological characteristics and epidermal detachment and necrosis, SJS and TEN are classified into a spectrum of diseases, abbreviated as SJS/TEN [1, 6, 7].

Based on the area of epidermal detachment (blister, erosion), Bastuji-Garin classified the spectrum of SJS/TEN into SJS, overlapping SJS/TEN and TEN. According to this classification, there are four subgroups as follows: (1) SJS with an epidermal detachment of less than 10% of the body area, red, pruritic, atypical target macules that are flat with normal skin; (2) overlap SJS/TEN with 10–30% epidermal detachment; (3) TEN with spots (spots) when the lesions are epidermal detachment over 30% of the body area, with red, widespread pruritus, atypical target macules; (4) TEN without spot with epidermal detachment lesions of more than 30% of the body area, no individual macules, no atypical target lesions [1, 2].
