A 52-year-old woman presents with painful erosions on her buccal mucosa and a violaceous, pruritic rash on her wrists and ankles. Histopathology shows a band-like lymphocytic infiltrate at the dermoepidermal junction with a saw-tooth pattern of the rete ridges. Which of the following is NOT a recognized feature of lichen planus?

Explanation

## Distinguishing Direct Immunofluorescence Patterns in Lichen Planus **Key Point:** Direct immunofluorescence (DIF) in lichen planus shows a **fibrinogen-rich band** along the basement membrane zone (BMZ), NOT granular IgA deposits. Granular IgA at the BMZ is the hallmark of IgA pemphigus or linear IgA disease, not lichen planus. ### Recognized Features of Lichen Planus | Feature | Description | Clinical Significance | |---------|-------------|----------------------| | **Wickham's striae** | Fine white reticular lines on oral lesions | Pathognomonic for oral LP | | **Koebner phenomenon** | New lesions at sites of trauma | Present in ~25% of LP patients | | **DIF pattern** | Fibrinogen-rich band at BMZ | Distinguishes LP from other lichenoid disorders | | **Nail involvement** | Thinning, ridging, pterygium unguis | Occurs in 10% of LP cases | | **Histology** | Band-like infiltrate, saw-tooth rete | Diagnostic hallmark | **High-Yield:** The correct DIF pattern in lichen planus is **fibrinogen deposition** (and sometimes fibrin) at the BMZ in a linear pattern. Granular IgA deposits indicate a different diagnosis entirely. **Clinical Pearl:** Oral lichen planus often presents with white reticular patterns (Wickham's striae) mixed with erosions and erythema. The combination of painful erosions + violaceous papules + oral involvement in this case is classic for LP. **Warning:** Do not confuse lichen planus DIF (fibrinogen-rich) with: - IgA pemphigus (granular IgA at BMZ) - Linear IgA disease (linear IgA at BMZ) - Pemphigus vulgaris (intercellular IgG) ### Why the Other Options Are Correct Features 1. **Wickham's striae** — Reticular white lines on oral mucosa, considered pathognomonic for oral LP. 2. **Koebner phenomenon** — Isomorphic response to trauma; occurs in a subset of LP patients. 3. **Nail involvement** — Pterygium unguis (pterygium formation), nail thinning, and longitudinal ridging are recognized manifestations of LP affecting the nail unit. [cite:Robbins 10e Ch 25]