A 52-year-old woman presents with a 3-month history of painful erosions on her buccal mucosa and gingiva. On examination, she has a lacy white pattern on the oral mucosa with underlying erythema. She also reports a pruritic rash on her flexural wrists and lower legs with violaceous, polygonal papules. Dermoscopy shows Wickham's striae. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Lichen Planus ### Key Diagnostic Features **Key Point:** Lichen planus is a chronic inflammatory disorder affecting skin and mucous membranes, characterized by the pathognomonic "6 P's": Planar (flat-topped), Purple, Polygonal, Papules, Plaques, and Pruritus. ### Clinical Presentation in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Oral involvement** | Lacy white (reticular) pattern with erosions | Pathognomonic for oral lichen planus; erosive variant indicates active disease | | **Skin lesions** | Violaceous, polygonal papules on flexural surfaces | Classic distribution and morphology | | **Dermoscopy** | Wickham's striae (white linear striations) | Gold standard dermoscopic finding; virtually diagnostic | | **Pruritus** | Present | One of the "6 P's"; distinguishes from other lichenoid disorders | | **Duration** | 3 months | Chronic course typical; may persist for years | ### Histopathology (Not Needed for Diagnosis but Confirmatory) **High-Yield:** Sawtooth appearance of the stratum spinosum with band-like lymphocytic infiltrate at the dermoepidermal junction (lichenoid interface dermatitis). Colloid bodies (Civatte bodies) in the basal layer. ### Clinical Pearls **Clinical Pearl:** Oral lichen planus carries a 1–2% risk of malignant transformation to oral squamous cell carcinoma, especially in erosive variants. Long-term surveillance is essential. **Clinical Pearl:** Lichen planus is often triggered by stress, certain medications (beta-blockers, NSAIDs, ACE inhibitors), and hepatitis C infection. Screen for these in every patient. ### Mnemonic for Lichen Planus Variants **Mnemonic:** CATER - **C**ommon (typical papules and plaques) - **A**trophic (thin, shiny lesions) - **T**uberous (nodular variant, rare) - **E**rosive (severe, especially oral) - **R**eticular (lacy, web-like pattern) ### Why Wickham's Striae Are Diagnostic Wickham's striae are fine white lines visible on dermoscopy that represent thickened stratum granulosum and hyperkeratosis. They are virtually pathognomonic for lichen planus and absent in mimics like pemphigus or erythema multiforme. ### Treatment Approach First-line: topical corticosteroids (potent for skin, intralesional for oral). Systemic therapy (oral corticosteroids, retinoids, or immunosuppressants) reserved for extensive or erosive disease.