Lentigo maligna — SCE Dermatology MCQ

Correct answer: D — Lentigo maligna

The correct answer is D, lentigo maligna. A slowly enlarging, irregular, asymmetric pigmented macule over 2 cm on chronically sun-exposed facial skin in an elderly patient, with dermoscopic asymmetric follicular pigmentation, rhomboidal structures and annular-granular grey dots, is the classic dermoscopic triad for lentigo maligna, an in-situ melanoma variant that must be actively excluded because delayed diagnosis risks progression to invasive lentigo maligna melanoma. These features reflect confluent atypical junctional melanocytes tracking down adnexal (follicular) structures, which is the histological correlate of the annular-granular and rhomboidal patterns. Absence of palpable infiltration does not exclude malignancy, as lentigo maligna is by definition confined to the epidermis; induration would instead suggest invasive transformation. Any lesion with these dermoscopic clues on the face of an older adult should prompt urgent biopsy or dermatology referral under suspected skin cancer pathways. Why the other options are wrong: B. Solar lentigo: Typically shows a fine, regular pigment network or fingerprint-like pattern with sharply demarcated moth-eaten borders, and lacks asymmetric follicular involvement or rhomboidal structures. C. Pigmented actinic keratosis: Shows a rosette sign, strawberry pattern with follicular openings surrounded by white-yellow keratotic scale, but not the pigmented rhomboidal or annular-granular pattern specific to melanocytic atypia. E. Seborrhoeic keratosis: Characterised by comedo-like openings, milia-like cysts and a sharply demarcated, symmetrical fissured or cerebriform surface, not asymmetric follicular pigmentation. A. Melasma: Presents as symmetrical, ill-defined tan-brown patches on the face, usually bilateral, with a fine pseudonetwork on dermoscopy and no follicular or rhomboidal structures. Key point: Asymmetric follicular pigmentation with annular-granular grey dots and rhomboidal structures on a facial macule in an older adult is pathognomonic enough to mandate excluding lentigo maligna, regardless of the absence of palpable infiltration.

Reference: DermNet NZ, Lentigo maligna and lentigo maligna melanoma dermoscopy, https://dermnetnz.org/topics/lentigo-maligna-and-lentigo-maligna-melanoma-dermoscopy