Bullous pemphigoid — SCE Dermatology MCQ

Correct answer: E — Linear IgG and C3 along the basement membrane

The correct answer is E, linear IgG and C3 along the basement membrane. This clinical picture (tense subepidermal bullae on erythematous plaques in an elderly patient, flexural and truncal distribution, peripheral eosinophilia, subepidermal blister with eosinophils on histology) is classic for bullous pemphigoid, and this presentation has been triggered by a DPP-4 inhibitor (sitagliptin), a recognised drug association. Direct immunofluorescence in bullous pemphigoid characteristically shows linear deposition of IgG and C3 along the dermo-epidermal basement membrane zone, reflecting autoantibodies against hemidesmosomal proteins BP180 and BP230. This linear BMZ pattern, ideally combined with indirect immunofluorescence on salt-split skin to localise antigens to the epidermal (roof) side, remains the diagnostic gold standard and is central to BAD guidance on bullous pemphigoid. Why the other options are wrong: A. Intercellular IgG throughout the epidermis: this is the fishnet pattern of pemphigus vulgaris, an intraepidermal blistering disease with acantholysis, not a subepidermal blister with eosinophils. D. Granular IgA in dermal papillae: this defines dermatitis herpetiformis, which presents with intensely itchy grouped vesicles on extensor surfaces and is linked to coeliac disease, not tense flexural bullae in the elderly. B. Shaggy fibrinogen along the basement membrane: this nonspecific fibrinogen deposition pattern is seen in lichen planus and other inflammatory dermatoses, not the immunoglobulin-driven autoimmune process described here. C. Perivascular IgM around superficial vessels: this pattern suggests a vasculitic or connective tissue process, not an autoimmune subepidermal blistering disorder. Key point: Tense bullae with subepidermal split and eosinophils in an elderly patient point to bullous pemphigoid, confirmed by linear IgG and C3 along the basement membrane zone on direct immunofluorescence.

Reference: British Association of Dermatologists' Guidelines for the Management of Bullous Pemphigoid 2012 (Venning VA et al., Br J Dermatol 2012;167(6):1200-1214), Diagnosis section on direct and indirect immunofluorescence findings. https://onlinelibrary.wiley.com/doi/10.1111/bjd.12072