This patient has bullous pemphigoid. Bullous pemphigoid is a chronic autoimmune blistering disease that predominantly affects older patients and presents with urticarial plaques with tense bullae often on the trunk and upper legs. Mucosal involvement can occur in 10% to 40% of patients. Associated peripheral eosinophilia also is common. Microscopic examination of a blistering lesion usually reveals the presence of eosinophils with the separation of tissue layers subepidermally causing tense bullae (as opposed to an intraepidermal blister that causes more flaccid bullae seen in pemphigus). Bullous pemphigoid is caused by autoantibodies directed against collagen XVII, which is bullous pemphigoid antigen 180 (BP180); direct immunofluorescence shows linear IgG deposition at the basement membrane zone.

Pemphigus foliaceus is a chronic autoimmune blistering disorder with autoantibodies against desmoglein 1. It results in very superficial intraepidermal, flaccid bullae, which are often easily ruptured, as opposed to the tense bullae in bullous pemphigoid. The lesions in pemphigus foliaceus often present as superficial erosions with a “corn flake”–like crust, as opposed to bullae. Mucosal involvement is also uncommon.

Pemphigus vulgaris is a chronic autoimmune blistering disorder that has autoantibodies predominantly against desmoglein 3, also resulting in flaccid bullae with mucous membrane involvement.

Toxic epidermal necrolysis is a severe medication reaction that results in fever, painful skin desquamation, and more than two mucosal surfaces that slough. It is not an autoimmune blistering disorder and would be unusual to present with pruritic edematous plaques.