Psoriasis — SCE Dermatology MCQ

Correct answer: D — PUVA (hand/foot soak)

The correct answer is D, PUVA (hand/foot soak). Palms and soles have a markedly thickened stratum corneum that limits penetration of UVB wavelengths, whereas the longer wavelength UVA, combined with a photosensitising psoralen soaked directly into the affected skin, achieves adequate dermal and epidermal penetration and clearance in this resistant site. Soak PUVA delivers psoralen topically to the hands and feet only, avoiding systemic photosensitisation, then UVA is applied locally, making it the modality of choice for localised palmoplantar disease. This approach is specifically recommended for palmoplantar psoriasis in phototherapy guidance because whole body or narrowband UVB units are far less effective at these acral sites. Why the other options are wrong: C. UVA1: UVA1 is used for deeper dermal disease such as morphoea and atopic dermatitis flares, not for hyperkeratotic palmoplantar psoriasis, and lacks the psoralen sensitisation needed to overcome the thick stratum corneum here. E. Broadband UVB: Broadband UVB has largely been superseded by narrowband UVB and, like NB-UVB, is poorly transmitted through thickened palmoplantar skin, giving inferior clearance rates. A. Narrowband UVB whole body: Whole body NB-UVB is first line for generalised plaque psoriasis but consistently shows reduced efficacy on palms and soles due to limited penetration through thick keratin. B. Excimer laser: Excimer laser (308 nm) is reserved for small, localised, stable plaques elsewhere on the body and is not practical or funded for widespread palmoplantar involvement. Key point: The thick stratum corneum of palms and soles requires topical psoralen sensitisation plus UVA (soak PUVA) to achieve adequate penetration, making it superior to UVB-based modalities for palmoplantar psoriasis.

Reference: Elmets CA et al. Joint AAD-NPF guidelines of care for the management and treatment of psoriasis with phototherapy, J Am Acad Dermatol 2019 (topical/soak PUVA recommended for palmoplantar psoriasis, principles compatible with UK phototherapy practice). https://www.jaad.org/article/S0190-9622(19)30637-1/fulltext