Due to pattern of epidermal denudement exposing an intact basement membrane zone, the possibility of an autoimmune vesiculobullous disorder targeting the basement membrane zone is raised. However the C3d and C4d stains were negative. Specifically linear deposits of C3d and C4d are not seen within the basement membrane zone. I would not be able to rule out an autoimmune process involving the basement membrane zone with cicatricial pemphigoid features especially if the targeted protein is located more superficially (i.e. above the salt split skin zone). That being said a more plausible basis however is one related to pralatrexate administration given the well known association of methotrexate-induced erosions.
Mönch S, Zimmo B, El Helou T, Colmegna I. Methotrexate-Induced Cutaneous Erosions. Arthritis Rheumatol. 2016 Jan;68(1):254.