ABD ABD Photodermatology and UV Effects 2 — Questions and Answers
Question 1: Porphyria cutanea tarda (PCT) is caused by deficiency of which enzyme in the heme biosynthesis pathway?
- Uroporphyrinogen decarboxylase (UROD) (Correct answer)
- Porphobilinogen deaminase
- Ferrochelatase
- Coproporphyrinogen oxidase
Correct answer: Uroporphyrinogen decarboxylase (UROD)
PCT results from deficiency of uroporphyrinogen decarboxylase (UROD), causing accumulation of uroporphyrins that absorb light and generate reactive oxygen species in the skin.
Question 2: Erythropoietic protoporphyria (EPP), the most painful photosensitivity disorder, is classically caused by mutations in which gene?
- FECH (ferrochelatase) (Correct answer)
- ALAS2 (X-linked EPP)
- PPOX (protoporphyrinogen oxidase)
- HMBS (hydroxymethylbilane synthase)
Correct answer: FECH (ferrochelatase)
Classic autosomal recessive EPP is caused by loss-of-function mutations in FECH (ferrochelatase), the terminal enzyme of heme synthesis, causing protoporphyrin IX accumulation.
Question 3: Which SPF value blocks approximately 97% of UVB radiation?
- SPF 15
- SPF 30 (Correct answer)
- SPF 50
- SPF 100
Correct answer: SPF 30
SPF 30 blocks approximately 97% of UVB radiation; SPF 15 blocks 93%, SPF 50 blocks 98%, and SPF 100 blocks 99%, showing diminishing returns at higher values.
Question 4: Chronic actinic dermatitis (CAD) is characterized by which predominant lymphocyte subset on histopathology?
- CD4+ T-helper cells
- CD8+ cytotoxic T cells (Correct answer)
- Regulatory T cells (Tregs)
- B cells (CD20+)
Correct answer: CD8+ cytotoxic T cells
CAD characteristically shows a CD8+ cytotoxic T-cell predominant infiltrate, which distinguishes it from other eczematous dermatoses that are predominantly CD4+.
Question 5: UVB radiation (280-315nm) is primarily responsible for photocarcinogenesis through which mechanism?
- Indirect DNA damage via reactive oxygen species
- Direct DNA damage forming cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (Correct answer)
- Protein crosslinking in the dermis
- Lipid peroxidation of cell membranes
Correct answer: Direct DNA damage forming cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts
UVB causes direct DNA damage by forming cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts between adjacent pyrimidines, producing C→T transition mutations.
Question 6: Phytophotodermatitis is caused by which class of photosensitizing compounds found in plants like lime and celery?
- Anthraquinones
- Furocoumarins (psoralens) (Correct answer)
- Chlorophyll derivatives
- Flavonoids
Correct answer: Furocoumarins (psoralens)
Phytophotodermatitis is caused by furocoumarins (psoralens) from plants in the Apiaceae and Rutaceae families, which upon UVA exposure cause phototoxic inflammation.
ABD ABD Photodermatology and UV Effects 2
Porphyria cutanea tarda (PCT) is caused by deficiency of which enzyme in the heme biosynthesis pathway?