A 48-year-old man with a 6-month history of pruritic papules on his wrists, forearms, and lower legs is diagnosed with cutaneous lichen planus on histopathology. Which is the most common site of involvement in cutaneous lichen planus?

Explanation

## Most Common Site of Cutaneous Lichen Planus **Key Point:** The flexor surfaces of the wrists and forearms are the most frequently affected sites in cutaneous lichen planus, followed by the lower legs and shins. These sites account for approximately 70–80% of cutaneous LP presentations. ### Anatomical Distribution in Cutaneous LP | Site | Frequency | Characteristics | |------|-----------|----------------| | Flexor wrists & forearms | 70–80% | Flat-topped papules, violaceous color, Wickham's striae | | Anterior lower legs/shins | 50–60% | Papules, plaques, may be hypertrophic | | Extensor elbows & knees | 20–30% | Less common than flexor surfaces | | Palms & soles | 10–15% | Rare; when present, indicates severe disease | | Trunk & abdomen | 15–25% | Secondary involvement; less common as primary site | | Genitalia | 10–15% | Erosive variant; often painful | **High-Yield:** The **flexor surfaces of the wrists and forearms** are the classic and most common site. This is a high-yield fact frequently tested in NEET PG. The lesions are characteristically **flat-topped, violaceous papules** with a shiny surface. ### Clinical Pearl The "**6 P's of Lichen Planus**" help recall the classic presentation: - **P**lanar (flat-topped) - **P**urple (violaceous) - **P**apules - **P**olygonal - **P**ruritic - **P**laces: flexor surfaces (wrists, forearms, lower legs) The flexor wrist involvement is so characteristic that it is often used as a diagnostic clue in clinical practice. ### Mnemonic **"LP Loves the Flexors"** — Lichen Planus most commonly affects flexor surfaces (wrists, forearms, anterior shins), not extensor surfaces. This distinguishes it from other papulosquamous disorders like psoriasis, which favors extensor surfaces. ### Pathophysiology The predilection for flexor surfaces is thought to be related to: 1. Increased friction and trauma in these areas 2. Higher density of lymphocytes in flexor skin 3. Possible role of irritants and allergens in flexor areas [cite:Robbins 10e Ch 25]