Dermoscopy — SCE Dermatology MCQ

Correct answer: C — Altered dermal collagen (fibrosis) visible due to birefringence under polarised light; associated with melanoma, BCC, dermatofibroma, and scars

The correct answer is C, altered dermal collagen (fibrosis) visible due to birefringence under polarised light, associated with melanoma, BCC, dermatofibroma, and scars. Shiny white lines (chrysalis or crystalline structures) are seen only with polarised dermoscopy because dermal collagen bundles are birefringent, scattering and randomising polarised light so fibrotic bands appear as bright white streaks, often oriented perpendicular to one another. This effect disappears under non polarised light or with immersion fluid, which is the key clue distinguishing them from other white structures. They raise suspicion for melanoma when combined with other atypical features, but are non specific, also occurring in benign fibrosing lesions like dermatofibroma and scars, so must be read in context. Why the other options are wrong: A. Blood vessels: vascular patterns (arborising, serpentine, dotted vessels) appear red or pink and linear or coiled, a distinct dermoscopic category unrelated to white collagen streaks. B. Keratin: keratin produces milia like cysts, comedo like openings, or yellow white structureless surface change typical of seborrhoeic keratosis, not polarisation dependent birefringent lines. D. Calcium deposits: calcification gives chalky white structureless clods (e.g. pilomatricoma), visible under both polarised and non polarised light, not a birefringence phenomenon. E. Melanin deposits: pigment causes brown, grey or blue colours from light absorption (network, dots, blotches), whereas shiny white lines result from light scattering by collagen, unrelated to melanin. Key point: shiny white lines are a polarised light only optical effect from birefringent dermal collagen (fibrosis), seen in melanoma, BCC, dermatofibroma and scars, and disappear with non polarised or contact dermoscopy.

Reference: DermNet NZ, Perpendicular white lines dermoscopy, 2019, https://dermnetnz.org/topics/perpendicular-white-lines-dermoscopy