Pityriasis Versicolor — SCE Dermatology MCQ

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EasyCutaneous InfectionPityriasis VersicolorSCE Dermatology

A 40-year-old man with Fitzpatrick skin type V presents with well-demarcated hypopigmented scaly patches on his trunk and upper limbs. He was previously treated with moderate potency topical corticosteroids with partial response. KOH scraping from the patches shows short hyphae and round spores described as a 'spaghetti and meatballs' pattern. What is the most appropriate diagnosis?

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Correct answer: C — Pityriasis versicolor

The correct answer is C, Pityriasis versicolor. This is a superficial Malassezia yeast infection producing well-demarcated, finely scaly hypo- or hyperpigmented macules on the trunk and upper limbs, more conspicuous in darker Fitzpatrick skin types because Malassezia produces azelaic acid derivatives that inhibit melanocyte tyrosinase. The KOH mount showing short curved hyphae with clusters of round yeast cells, the classic spaghetti and meatballs pattern, is diagnostic and distinguishes it from other causes of hypopigmentation. Partial response to a moderate potency steroid fits, since steroids reduce associated inflammation and scale but do not eradicate the fungal element, explaining the incomplete improvement described. Why the other options are wrong: A. Pityriasis alba: This presents as ill-defined, faintly scaly hypopigmented patches, typically on the face of children or young people, and KOH microscopy is negative because it is an eczematous, not fungal, process. D. Vitiligo: Produces sharply demarcated, completely depigmented (not hypopigmented) macules due to melanocyte destruction, with no scale and a negative KOH scraping. E. Mycosis fungoides: Hypopigmented cutaneous T-cell lymphoma can mimic this but requires skin biopsy showing atypical lymphocytes with epidermotropism; KOH would be negative for fungal elements. B. Post-inflammatory hypopigmentation: Follows a prior inflammatory dermatosis at the same site, has no scale, and shows no hyphae or spores on microscopy. Key point: A positive KOH scraping showing spaghetti and meatballs hyphae and spores confirms Malassezia infection and clinches pityriasis versicolor over other causes of hypopigmentation.

Reference: DermNet NZ, Pityriasis versicolor (Tinea versicolor), reviewed 2023, https://dermnetnz.org/topics/pityriasis-versicolor