Phototherapy — SCE Dermatology MCQ

Correct answer: B — The lowest UV dose that produces just perceptible erythema 24 hours after exposure

Option B (the lowest UV dose that produces just perceptible erythema 24 hours after exposure) is correct. MED testing is a mandatory pre-treatment step in UK phototherapy services and involves exposing small patches of unexposed skin to a graded series of UV doses; the reading is taken at 24 hours because erythema from UVB peaks at this time point. The MED defines the threshold of the patient's individual photosensitivity and is used to calculate the safe starting dose for narrowband UVB (typically 50 to 70 percent of MED), then incremented according to protocol. This individualised approach accounts for variation in skin type, recent sun exposure and photosensitising drugs, minimising the risk of burning while ensuring an adequate therapeutic dose is reached efficiently. Why the other options are wrong: E. Blistering: this represents a severe phototoxic burn reaction (grade well above minimal erythema) and indicates gross overdose, not the threshold dose sought for calibration. D. Dose needed to clear psoriasis: this describes a cumulative therapeutic endpoint over a full course, not a single calibration measurement taken at 24 hours before treatment starts. A. Permanent skin damage: MED testing deliberately uses sub-injurious doses; causing permanent damage would be a serious adverse outcome, not the intended endpoint. C. Maximum safe lifetime UV dose: there is no such single defined dose in phototherapy protocols; cumulative exposure is instead monitored via total number of treatments and lifetime course limits, not an MED value. Key point: MED is the threshold dose producing just visible erythema at 24 hours, used only to calculate a safe starting dose for phototherapy, not a treatment endpoint or damage threshold.

Reference: British Association of Dermatologists, Phototherapy Service Standards (General Recommendation R3, MED determination before starting a course of phototherapy); cited in Britton J, Verma P. 2024 results of a United Kingdom and Ireland phototherapy minimal erythema dose survey. Clinical and Experimental Dermatology 2025;50(4):866-867. https://doi.org/10.1093/ced/llae444