A 68-year-old man presents with tense bullae on the flexural aspects of both forearms and inner thighs for 3 months. Histopathology shows subepidermal bulla with intact basal layer and eosinophilic infiltrate. Direct immunofluorescence reveals linear IgG and C3 deposits along the basement membrane zone. What is the most common site of involvement in bullous pemphigoid?

## Most Common Sites in Bullous Pemphigoid **Key Point:** Bullous pemphigoid characteristically affects **flexural surfaces and areas of friction** — particularly the inner thighs, axillae, antecubital fossae, lower abdomen, and groin. ### Anatomical Predilection | Site | Frequency | Clinical Feature | |------|-----------|------------------| | Flexural areas (thighs, axillae, antecubital fossae) | Most common (>70%) | Tense bullae, post-inflammatory hyperpigmentation | | Lower abdomen and groin | Common | Bullae on areas of friction | | Palms and soles | Rare | Unlike pemphigus vulgaris | | Oral mucosa | Rare (<10%) | Mild involvement, not primary site | | Scalp and face | Uncommon | Atypical presentation | **High-Yield:** The predilection for **flexural and intertriginous areas** is a key distinguishing feature of bullous pemphigoid. This contrasts sharply with pemphigus vulgaris, which favors oral mucosa and face. ### Why Flexural Areas? 1. **Mechanical trauma** — friction and pressure increase antigen expression and blister formation 2. **Moisture and maceration** — intertriginous environment favors lesion development 3. **Increased basement membrane zone antigen density** — BP180 and BP230 are more concentrated in areas of stress **Clinical Pearl:** Post-inflammatory hyperpigmentation is common in flexural bullous pemphigoid lesions, especially in darker skin types, and may persist for months after healing. **Mnemonic: "BP = Bullae in flexural Planes"** — remember flexural predilection as the hallmark site distribution. [cite:Robbins 10e Ch 25]