A 68-year-old man presents with a 6-week history of pruritic lesions on his flexural surfaces and lower abdomen. On examination, the lesions marked **A** in the diagram are firm, intact, and do not rupture easily when gentle pressure is applied. The surrounding skin shows erythema and urticarial changes. Nikolsky sign is negative. A biopsy of perilesional normal skin shows linear IgG and C3 deposition along the basement membrane zone. Which of the following is the most likely diagnosis?

Explanation

## Why Bullous pemphigoid is right The clinical presentation of **tense (firm, intact) bullae** marked **A** that do not rupture easily, on an erythematous background in an elderly patient (>60 years), with negative Nikolsky sign and linear IgG + C3 along the basement membrane zone on direct immunofluorescence, is pathognomonic for bullous pemphigoid. The mechanism involves IgG autoantibodies against hemidesmosomal proteins (BP180/BPAG2 and BP230/BPAG1), causing subepidermal blister formation with an intact roof, which accounts for the characteristic firmness and resistance to rupture. Mucosal involvement is rare, distinguishing it from pemphigus vulgaris (Robbins 10e Ch 25; Harrison 21e Ch 56). ## Why each distractor is wrong - **Pemphigus vulgaris**: Presents with flaccid bullae that rupture easily (not tense), positive Nikolsky sign, and oral mucosal involvement is typically the first manifestation. Direct IF shows intercellular IgG (acantholytic pattern), not linear basement membrane deposition. Mechanism is intraepidermal acantholysis, not subepidermal split. - **Dermatitis herpetiformis**: Associated with celiac disease, presents with intensely pruritic grouped vesicles (not large bullae), and direct IF shows granular IgA deposition at the dermal-epidermal junction, not linear IgG + C3. Typically affects extensor surfaces and buttocks. - **Linear IgA disease**: While it can present with tense bullae and linear IgA deposition along the basement membrane, it is more common in children and young adults (not predominantly elderly), and is associated with drugs (especially vancomycin) rather than the typical elderly demographic with neurologic comorbidities seen in bullous pemphigoid. **High-Yield:** Tense bullae + elderly + negative Nikolsky + linear IgG/C3 at BMZ = bullous pemphigoid; flaccid bullae + positive Nikolsky + oral lesions = pemphigus vulgaris. [cite: Robbins 10e Ch 25; Harrison 21e Ch 56]