A 68-year-old Indian woman presents with a 3-week history of tense, fluid-filled blisters on her lower abdomen, inner thighs, and flexural areas. The blisters are intact, non-flaccid, and do not rupture easily. She denies oral involvement. On examination, there is no mucosal involvement and the surrounding skin appears erythematous but without urticaria. Nikolsky sign is negative. Histopathology shows subepidermal blister with eosinophil-rich infiltrate. Direct immunofluorescence (DIF) reveals linear IgG and C3 deposits along the basement membrane zone. What is the most likely diagnosis?

Explanation

## Diagnosis: Bullous Pemphigoid ### Clinical Presentation **Key Point:** Bullous pemphigoid (BP) is characterized by large, tense, intact blisters that do not rupture easily, typically affecting flexural areas, lower abdomen, and inner thighs in elderly patients. ### Histopathology & Immunofluorescence **High-Yield:** The hallmark finding is: - **Subepidermal blister** with eosinophil-rich infiltrate (eosinophils are prominent in BP) - **Linear IgG and C3 deposits along the basement membrane zone** on DIF — this is pathognomonic for BP ### Distinguishing Features from Other Bullous Disorders | Feature | Bullous Pemphigoid | Pemphigus Vulgaris | Linear IgA Disease | Dermatitis Herpetiformis | |---------|-------------------|-------------------|-------------------|------------------------| | **Blister level** | Subepidermal | Intraepidermal (acantholysis) | Subepidermal | Subepidermal | | **Blister integrity** | Tense, intact | Flaccid, ruptures easily | Variable | Vesicles (smaller) | | **Oral involvement** | Rare | Common (>90%) | Possible | No | | **Nikolsky sign** | Negative | Positive | Negative | Negative | | **DIF pattern** | Linear IgG + C3 at BMZ | Intercellular IgG ("tombstone") | Linear IgA at BMZ | Granular IgA at dermal–epidermal junction | | **Age of onset** | Elderly (>60 years) | Any age | Any age | Young adults | | **Eosinophils on histology** | Prominent | Absent | Variable | Absent | ### Clinical Pearl **Key Point:** Negative Nikolsky sign + negative oral involvement + linear IgG/C3 at BMZ = BP, not pemphigus. ### Pathophysiology **Mnemonic: BP-180 and BP-230** — Bullous pemphigoid autoantibodies target hemidesmosomal proteins (BP180 and BP230), leading to complement activation and subepidermal blister formation. ### Management Overview - Mild: Topical corticosteroids - Moderate to severe: Systemic corticosteroids (prednisolone 0.5–1 mg/kg/day) - Steroid-sparing agents: Azathioprine, mycophenolate mofetil [cite:Robbins 10e Ch 25]