Lichen Nitidus — SCE Dermatology MCQ

Correct answer: C — Lichen nitidus

The correct answer is C, Lichen nitidus. This condition is characterised by multiple, tiny (1 to 3 mm), sharply demarcated, flat-topped, skin-coloured, shiny papules that favour the forearms, trunk and genitalia, exactly as described in this patient, and lesions are typically asymptomatic. The pathognomonic histological finding is a well-circumscribed dermal infiltrate of lymphocytes and histiocytes confined to a widened papillary dermis, "clutched" on either side by elongated, downward-projecting rete ridges, giving the classic ball-and-claw appearance. Lichen nitidus is a papular eruption seen mainly in children and young adults and is generally self-limiting, needing no treatment beyond reassurance in most cases. The combination of pinpoint shiny papules plus this specific papillary dermal granuloma pattern is unique to lichen nitidus and distinguishes it from other papulosquamous and lichenoid dermatoses. Why the other options are wrong: A. Guttate psoriasis: presents with small erythematous scaly papules and plaques, often post-streptococcal, with histology showing psoriasiform hyperplasia, parakeratosis and neutrophilic microabscesses, not a papillary dermal lymphohistiocytic ball-and-claw infiltrate. B. Flat warts: caused by HPV, show verrucous acanthosis with koilocytosis and hyperkeratosis on histology, a viral epidermal change rather than a circumscribed dermal granulomatous infiltrate. E. Lichen planus: papules are typically violaceous, polygonal and intensely pruritic with a saw-tooth pattern of rete ridges, band-like lymphocytic infiltrate at the dermoepidermal junction, and Civatte bodies, differing from the tiny non-pruritic papules and ball-and-claw histology here. D. Keratosis pilaris: produces rough follicular keratotic papules due to follicular plugging, with histology showing follicular hyperkeratosis, not a papillary dermal granuloma. Key point: Tiny, shiny, flat-topped asymptomatic papules with a papillary dermal lymphohistiocytic infiltrate flanked by elongated rete ridges (ball-and-claw pattern) is diagnostic of lichen nitidus.

Reference: Primary Care Dermatology Society (PCDS), Lichen nitidus, https://www.pcds.org.uk/clinical-guidance/lichen-nitidus