Retinoid Mechanism — SCE Dermatology MCQ

Correct answer: D — Retinoids bind to nuclear retinoid receptors (RAR/RXR), normalising keratinocyte differentiation and proliferation, reducing cornification, and modulating immune function

The correct answer is D: Retinoids bind to nuclear retinoid receptors (RAR/RXR), normalising keratinocyte differentiation and proliferation, reducing cornification, and modulating immune function. Acitretin is a second-generation oral retinoid that acts as a ligand for RAR (alpha, beta, gamma) and RXR nuclear receptors, which function as transcription factors regulating genes controlling keratinocyte growth, differentiation and cornification. This corrects the hyperproliferative, poorly differentiated epidermis characteristic of psoriasis and reduces scaling, while also exerting immunomodulatory effects on cytokine production and antigen-presenting cell function. NICE guidance recommends acitretin as a systemic option for psoriasis, particularly pustular and palmoplantar disease, reflecting this receptor-mediated mode of action rather than a targeted immune-cytokine block. Why the other options are wrong: B. By blocking histamine: Histamine blockade is the mechanism of antihistamines used for pruritus, not a mechanism relevant to psoriasis pathogenesis or retinoid pharmacology. A. By killing immune cells: Retinoids do not act as cytotoxic or immune cell depleting agents; they modulate gene transcription via nuclear receptors rather than causing immune cell death. E. By blocking TNF: TNF blockade describes the mechanism of biologic agents such as adalimumab or etanercept, which are structurally and mechanistically unrelated to retinoids. C. By inhibiting IL-17: IL-17 inhibition is the mechanism of biologics such as secukinumab and ixekizumab, not of oral retinoids, which act upstream at the nuclear receptor and differentiation level rather than on a single cytokine pathway. Key point: Acitretin works through nuclear RAR/RXR receptor binding to normalise keratinocyte differentiation and cornification, distinguishing it mechanistically from cytokine-targeted biologic therapies for psoriasis.

Reference: NICE Guideline CG153, Psoriasis: assessment and management, Recommendations on systemic therapy including acitretin, https://www.nice.org.uk/guidance/cg153/chapter/Recommendations