Xeroderma Pigmentosum — SCE Dermatology MCQ

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HardSkin CancerXeroderma PigmentosumSCE Dermatology

A 55-year-old man presents with xeroderma pigmentosum. He has extreme photosensitivity, freckling from infancy, and has developed multiple skin cancers (BCCs, SCCs, and melanoma) in his 30s. What is the underlying genetic defect?

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Correct answer: D — Defective nucleotide excision repair (NER) of UV-induced DNA damage

Xeroderma pigmentosum (XP) is an autosomal recessive disorder caused by defective nucleotide excision repair (NER) of UV-induced DNA damage (pyrimidine dimers). Eight complementation groups (XPA-XPG plus variant XPV) exist, each affecting different NER proteins. Without functional DNA repair, UV mutations accumulate rapidly leading to extreme photocarcinogenesis — 1000-fold increased skin cancer risk and median age of first NMSC at 8-10 years. Management is rigorous lifelong photoprotection (UV-blocking film on windows, protective clothing, high-SPF broad-spectrum sunscreen), regular skin surveillance with low threshold for excision, and retinoid chemoprevention (oral Acitretin may reduce NMSC incidence). Some subtypes also develop neurodegeneration.

Reference: BAD 2019; NICE Rare Diseases