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 erosive lesions. She also complains of a pruritic rash on her flexural wrists and lower legs, with shiny, flat-topped purple papules arranged in a linear pattern. Dermoscopy of the skin lesions shows a characteristic reticular pattern with white lines. She denies any systemic symptoms. What is the most likely diagnosis?

Explanation

## Diagnosis: Lichen Planus with Oral Involvement ### 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. **Skin lesions**: Shiny, flat-topped purple papules (the "6 Ps" — Purple, Planar, Papules, Pruritic, Polygonal, Plaques) 3. **Distribution**: Flexural surfaces (wrists, lower legs) — typical predilection sites ### Diagnostic Features | Feature | Lichen Planus | Significance | |---------|---------------|-------------| | **Wickham's striae** | White reticular lines on oral mucosa | Pathognomonic finding | | **Lesion morphology** | Flat-topped, shiny, purple papules | Characteristic appearance | | **Dermoscopy** | Reticular white lines (Wickham's striae pattern) | Non-invasive confirmation | | **Distribution** | Flexural surfaces, oral mucosa, genitals | Typical sites | | **Pruritus** | Often present | Distinguishes from other conditions | **Key Point:** Oral lichen planus occurs in 50–70% of patients with cutaneous lichen planus. The combination of oral Wickham's striae and characteristic skin lesions is diagnostic. ### Pathophysiology **High-Yield:** Lichen planus is a T-cell–mediated autoimmune disorder targeting basal keratinocytes. Histology shows a characteristic "saw-tooth" appearance of the rete ridges with a dense band-like lymphocytic infiltrate in the superficial dermis. ### Clinical Pearl The linear arrangement of papules (Koebner phenomenon) and the pruritic nature help distinguish lichen planus from other erosive oral conditions. Oral erosive lichen planus carries a small risk of malignant transformation (0.5–2%). ### Management Approach 1. **Topical**: Corticosteroid creams/ointments for skin; topical corticosteroids or tacrolimus for oral lesions 2. **Systemic**: Oral corticosteroids or retinoids for severe erosive disease 3. **Monitoring**: Regular follow-up, especially for oral lesions, to screen for malignant transformation **Mnemonic:** **6 Ps of Lichen Planus** — Purple, Planar, Papules, Pruritic, Polygonal, Plaques