Erythroderma and Bullae in a Psoriatic Patient: Navigating Diagnostic Bias.

Erythroderma, defined as erythema and scaling involving almost the entire body surface area, is a potentially life-threatening dermatologic condition that may be idiopathic or secondary to inflammatory diseases such as psoriasis or atopic dermatitis, drug reactions, autoimmune blistering disorders, or malignancy. Bullous pemphigoid (BP), a common autoimmune blistering disorder in older adults, rarely presents with erythroderma, and its coexistence with psoriasis is uncommon. We report the case of a 62-year-old man with long-standing psoriasis who presented with one month of progressive pruritus and erythroderma following recent furosemide use, with worsening of his eruption despite a prednisone taper prescribed at an outside urgent care. He was initially treated with cyclosporine for a presumed psoriasis flare but developed numerous tense bullae shortly after admission. Biopsies obtained at presentation demonstrated subepidermal bullae with eosinophils on hematoxylin and eosin staining, and linear IgG and C3 along the basement membrane on direct immunofluorescence, confirming BP. Discontinuation of furosemide and initiation of systemic corticosteroids, followed by adjunctive therapies, led to complete remission. This case highlights the diagnostic challenges of erythroderma and the risk of anchoring bias in patients with pre-existing dermatoses. A thorough medication history, early biopsy, and immunopathologic evaluation are essential to avoid delays in recognizing atypical presentations of BP.