A 68-year-old Indian woman presents with a 3-week history of tense, fluid-filled blisters on the flexural aspects of both forearms and inner thighs. The blisters do not rupture easily and the surrounding skin appears normal. She denies oral involvement. On examination, a positive Nikolsky sign is absent. Laboratory investigations show normal complete blood count and liver function tests. Direct immunofluorescence (DIF) of perilesional skin reveals linear IgG and C3 deposits along the basement membrane zone. What is the most likely diagnosis?

Explanation

## Clinical Presentation **Key Point:** Bullous pemphigoid (BP) is characterized by large, tense, non-flaccid blisters that do NOT rupture easily, arising on normal or erythematous skin. **High-Yield:** The hallmark of BP is the **absence of mucosal involvement** and **negative Nikolsky sign**, which distinguishes it from pemphigus vulgaris. ## Diagnostic Criteria | Feature | Bullous Pemphigoid | Pemphigus Vulgaris | |---------|-------------------|--------------------| | Blister type | Tense, intact | Flaccid, ruptures easily | | Nikolsky sign | Negative | Positive | | Oral involvement | Rare | Common (90%) | | DIF pattern | Linear IgG + C3 at BMZ | Intercellular IgG (acantholysis) | | Histology level | Subepidermal | Intraepidermal | | Age of onset | Elderly (>60 years) | Variable | ## Immunofluorescence Pattern **Clinical Pearl:** Direct immunofluorescence showing **linear IgG and C3 deposits along the basement membrane zone** is pathognomonic for bullous pemphigoid. This reflects autoantibodies against BP180 (collagen XVII) and BP230 antigens in the hemidesmosomes. **Key Point:** The **linear pattern** at the dermal-epidermal junction is the gold standard for BP diagnosis and is NOT seen in pemphigus (which shows intercellular staining). ## Pathophysiology ```mermaid flowchart TD A[Autoimmune attack on BP180/BP230]:::outcome --> B[Antibody-mediated complement activation] B --> C[Destruction of hemidesmosomes] C --> D[Subepidermal blister formation] D --> E[Tense, intact blisters] E --> F[Negative Nikolsky sign]:::outcome ``` ## Why This Patient Has BP 1. **Age >60 years** — classic presentation in elderly 2. **Tense, non-rupturing blisters** — pathognomonic for BP 3. **Negative Nikolsky sign** — rules out pemphigus 4. **No oral involvement** — typical for BP 5. **Linear IgG + C3 at BMZ** — diagnostic gold standard **Mnemonic:** **TENT-BP** = **T**ense blisters, **E**lderly, **N**egative Nikolsky, **T**hick basement membrane (subepidermal), **BP** = Bullous Pemphigoid.