A 42-year-old woman presents with a 3-year history of erythematous scaly plaques on her cheeks, nose, and scalp. Examination reveals well-demarcated lesions with follicular plugging and, in older lesions, the structure marked **C** (atrophic central scarring with surrounding dyspigmentation). Skin biopsy shows epidermal atrophy, vacuolar interface dermatitis, and periappendageal lymphocytic infiltrate. Direct immunofluorescence demonstrates granular IgG and C3 at the dermoepidermal junction in lesional skin. Which of the following clinical features is the MOST important distinguishing characteristic of discoid lupus erythematosus (DLE) that is demonstrated by the scarring process marked **C**?

Explanation

## Why "Permanent scarring alopecia on scalp lesions, distinguishing DLE from systemic lupus erythematosus which causes non-scarring alopecia" is right The structure marked **C** — atrophic central scarring — is the pathognomonic feature that distinguishes discoid lupus erythematosus from systemic lupus erythematosus. When DLE lesions occur on the scalp, the chronic atrophy and scarring process destroys hair follicles, resulting in **permanent scarring alopecia**. This is a classic distinguishing feature: SLE causes non-scarring alopecia (telogen effluvium or anagen effluvium), whereas DLE causes irreversible scarring alopecia due to the destructive nature of the central atrophic lesion. This distinction is critical for diagnosis and prognostication (Robbins 10e Ch 6; Bolognia Dermatology 4e). ## Why each distractor is wrong - **Development of hypo-pigmentation centrally with hyper-pigmentation at the periphery, indicating progression to systemic lupus**: While dyspigmentation (hypo- and hyper-pigmentation) is indeed part of the scarring lesion marked **C**, this finding does NOT indicate progression to SLE. Dyspigmentation is a feature of chronic DLE lesions and can occur in localized DLE without systemic involvement. The question asks for the MOST important distinguishing characteristic, which is the scarring alopecia pattern, not dyspigmentation alone. - **Epidermal atrophy with follicular plugging that responds to topical corticosteroids alone without systemic therapy**: Epidermal atrophy and follicular plugging are indeed histologic features of DLE, but they are not specific to DLE and do not distinguish it from SLE. Moreover, extensive or refractory DLE often requires systemic therapy (hydroxychloroquine, methotrexate) beyond topical corticosteroids. This distractor conflates histology with treatment response. - **Central scarring with preserved sensation, indicating involvement of dermal but not subcutaneous structures**: This is anatomically misleading. DLE is a dermatologic condition affecting the dermis and epidermis, but the question is not about sensory preservation or depth of involvement. The scarring in DLE is destructive to skin appendages (hair follicles), not merely a depth-based distinction. This distractor introduces irrelevant neurologic language. **High-Yield:** Permanent scarring alopecia on the scalp is the hallmark of DLE and the key feature that separates it from SLE, which causes non-scarring alopecia. [cite: Robbins 10e Ch 6; Bolognia Dermatology 4e]