A 68-year-old man presents with tense, fluid-filled bullae on the flexural surfaces of the forearms and inner thighs. Histopathology shows subepidermal blistering with intact basal layer. Which is the most common site of involvement in bullous pemphigoid?

Explanation

## Most Common Sites in Bullous Pemphigoid **Key Point:** The lower abdomen, inner thighs, flexural surfaces of forearms, and lower legs are the most frequently affected sites in bullous pemphigoid, accounting for >80% of cases. ### Site Distribution | Site | Frequency | Characteristics | | --- | --- | --- | | **Lower abdomen & inner thighs** | Most common (>80%) | Flexural, intertriginous areas | | Forearms (flexural) | Very common | Often bilateral | | Lower legs | Common | Especially shins | | Oral mucosa | Rare (<10%) | Unlike pemphigus vulgaris | | Scalp & face | Uncommon | When present, often mild | | Dorsal hands/feet | Uncommon | Trauma-prone sites | ### Clinical Pearl **High-Yield:** Bullous pemphigoid shows a predilection for **flexural and intertriginous areas** — sites subject to friction and moisture. This is in stark contrast to pemphigus vulgaris, which commonly involves the oral mucosa first. ### Pathophysiology The preferential involvement of flexural areas is thought to relate to: 1. Increased skin friction and maceration in these zones 2. Higher local humidity and occlusion 3. Mechanical trauma triggering blister formation at sites of pre-existing autoimmune basement membrane damage **Warning:** Do not confuse with pemphigus vulgaris, which characteristically begins in the oral mucosa (>90% of cases) and spreads to skin. [cite:Robbins 10e Ch 25]