In a phototherapy unit, dose escalation protocols differ between narrow-band UVB and PUVA. The reason PUVA doses are generally incremented less frequently than narrow-band UVB doses is that PUVA-induced erythema:
- A Peaks at 48 to 72 hours, so earlier escalation risks cumulative burning ✓
- B Peaks within 6 hours of exposure, allowing rapid titration
- C Never occurs in darker skin phototypes
- D Is prevented entirely by concomitant topical corticosteroids
Correct answer: A. Peaks at 48 to 72 hours, so earlier escalation risks cumulative burning
Explanation
Psoralen intercalates into DNA and, after UVA activation, forms monoadducts and cross-links, producing a delayed erythematous reaction that peaks around 48 to 72 hours, much later than UVB erythema which peaks near 24 hours. Because the full burn potential of a PUVA dose is not apparent for two to three days, escalating too quickly causes severe cumulative phototoxicity. Darker phototypes still develop PUVA erythema, only at higher thresholds, and steroids do not abolish the reaction.
Reference: Rook's Textbook of Dermatology, 9th ed.
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