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 reticular pattern of white lines (Wickham's striae) on the oral mucosa, along with areas of ulceration. She also reports pruritic, violaceous, flat-topped papules on her wrists and ankles. Dermoscopy of the skin lesions shows a characteristic reticular pattern with Wickham's striae. What is the most likely diagnosis?

## Diagnosis: Lichen Planus ### Clinical Presentation This patient presents with the classic triad of lichen planus: 1. **Oral involvement** — reticular white lines (Wickham's striae) with erosions on buccal mucosa and gingiva 2. **Cutaneous lesions** — violaceous, flat-topped papules on flexural surfaces (wrists, ankles) 3. **Dermoscopic findings** — characteristic reticular pattern with Wickham's striae ### Key Diagnostic Features **Key Point:** Lichen planus is a T-cell mediated autoimmune disorder affecting the basal keratinocytes. The "6 P's" mnemonic helps recall cutaneous features: **Mnemonic: 6 P's of Lichen Planus** — **P**urple, **P**lanar (flat-topped), **P**apules, **P**olygonal, **P**lain (non-scaly or minimally scaly), **P**ruritic ### Oral vs. Cutaneous Lichen Planus | Feature | Oral LP | Cutaneous LP | | --- | --- | --- | | Appearance | Reticular, erosive, atrophic | Violaceous papules, plaques | | Wickham's Striae | Prominent white lines | Visible on dermoscopy | | Pain | Often painful/erosive | Pruritic | | Common sites | Buccal mucosa, gingiva, tongue | Wrists, ankles, genitals | | Malignant potential | 0.5–5% risk of SCC | <1% | ### Pathology & Mechanism **High-Yield:** Histopathology shows: - Dense band-like lymphocytic infiltrate at dermoepidermal junction (lichenoid interface dermatitis) - Basal cell degeneration (liquefactive necrosis) - Saw-tooth hyperkeratosis - Civatte bodies (apoptotic keratinocytes) ### Dermoscopic Pattern **Clinical Pearl:** The characteristic **Wickham's striae** (white reticular pattern) are pathognomonic when seen on dermoscopy. This represents thickened stratum corneum in the grooves of the papillary surface. ### Treatment Approach ```mermaid flowchart TD A[Lichen Planus Diagnosis Confirmed]:::outcome --> B{Severity & Location?}:::decision B -->|Mild cutaneous| C[Topical corticosteroids]:::action B -->|Moderate cutaneous| D[Intralesional steroids + Topical agents]:::action B -->|Severe/Extensive| E[Systemic corticosteroids or Retinoids]:::action B -->|Oral involvement| F[Topical steroids + Antimicrobials for erosions]:::action F --> G[Monitor for malignant transformation]:::action ``` ### Monitoring **Warning:** Oral lichen planus carries a 0.5–5% risk of malignant transformation to squamous cell carcinoma. Regular surveillance and biopsy of any atypical areas is essential.