A 31-year-old woman presents with an 18-month history of slowly enlarging, itchy red patches on her cheeks and bridge of the nose, worse after sun exposure. Examination reveals well-demarcated erythematous plaques with central atrophy and hypopigmentation. When the coarse scale overlying each plaque (marked **A** in the diagram) is peeled away, follicular spines are revealed on its undersurface. Skin biopsy confirms chronic cutaneous lupus erythematosus (discoid type). Which of the following best describes the pathologic significance of the adherent keratotic scale marked **A** in establishing this diagnosis?
A. It demonstrates psoriasiform hyperplasia with parakeratosis, indicating that this is a variant of plaque psoriasis rather than lupus
B. It represents the carpet-tack sign, a highly specific morphologic feature of discoid lupus erythematosus caused by follicular plugging and keratotic material adherent to follicular ostia
C. It indicates seborrheic keratosis with secondary inflammation, requiring differentiation from lupus by absence of interface dermatitis on biopsy
D. It reflects chronic photodamage with actinic keratosis formation, which is the primary pathologic process in sun-exposed cutaneous lupus
Explanation
Why the carpet-tack sign is right
The adherent keratotic scale marked A, when peeled away to reveal follicular spines on its undersurface, is the pathognomonic carpet-tack sign of discoid lupus erythematosus. This sign results from follicular plugging with keratotic material that remains adherent to the scale, creating a distinctive appearance. According to Werth VP (NEJM 2023) and EULAR CLE guidance 2023, this morphologic finding is one of the most specific diagnostic clues for DLE and directly supports the histopathologic findings of follicular plugging and interface dermatitis seen on biopsy.
Why each distractor is wrong
Seborrheic keratosis with secondary inflammation: While seborrheic keratosis presents with adherent scale, it lacks the follicular plugging pattern and does not produce the carpet-tack sign. Interface dermatitis is absent in seborrheic keratosis and present in DLE, making this distinction clear on biopsy.
Chronic photodamage with actinic keratosis: Although sun exposure worsens DLE, actinic keratosis presents with a different scale pattern (hyperkeratotic but not follicular-based) and lacks the characteristic carpet-tack sign. Actinic keratosis shows solar elastosis on biopsy, not the lupus band or interface dermatitis of DLE.
Psoriasiform hyperplasia with parakeratosis (plaque psoriasis): Psoriasis presents with silvery scale and parakeratosis on biopsy, not the follicular plugging and keratotic spines of the carpet-tack sign. Psoriasis lacks interface dermatitis, basement membrane thickening, and the lupus band seen in DLE.
High-YieldNEET PG
The carpet-tack sign—follicular spines revealed when the adherent keratotic scale is peeled away—is the single most specific morphologic clue for discoid lupus erythematosus and should prompt immediate consideration of DLE in the differential diagnosis.
