A 24-year-old male athlete from Mumbai presents with multiple oval hypopigmented and erythematous macules with fine branny scale over his chest and back that have been present for 3 months. Wood's lamp examination shows yellow-green fluorescence. KOH mount of the scale reveals the classic "spaghetti and meatballs" pattern. The condition marked **D** in the diagram is responsible for this presentation. Which of the following best describes the pathogenic mechanism by which this organism causes hypopigmentation in this patient?

Explanation

## Why "Production of azelaic acid that competitively inhibits tyrosinase and suppresses melanocyte function" is right The clinical anchor states explicitly that **hypopigmentation in pityriasis versicolor is caused by azelaic acid produced by Malassezia species — a competitive inhibitor of tyrosinase that suppresses melanocyte function**. This is the gold-standard mechanism taught in Bolognia Dermatology and IADVL. The patient's presentation (hypopigmented macules, "spaghetti and meatballs" on KOH, yellow-green Wood's lamp fluorescence) is pathognomonic for Malassezia furfur/globosa infection, and the question tests the student's understanding of WHY hypopigmentation occurs — not just that it does. ## Why each distractor is wrong - **Direct destruction of melanocytes through fungal keratinolytic enzymes**: Malassezia does not destroy melanocytes; it is a superficial stratum corneum infection. The organism does not invade deeper dermis or target melanocytes directly. This is a plausible-sounding mechanism but contradicts the pathophysiology. - **Increased production of enlarged melanosomes leading to pigment deposition**: This mechanism actually causes HYPERPIGMENTATION (not hypopigmentation) in tinea versicolor, as stated in the anchor. This distractor tests whether students confuse the two color variants. - **Competitive inhibition of DOPA oxidase in the melanin synthesis pathway**: While DOPA oxidase is involved in melanin synthesis, the anchor specifically names TYROSINASE as the enzyme inhibited by azelaic acid. DOPA oxidase is a different enzyme in the pathway, and this represents a common confusion point in biochemistry. **High-Yield:** Azelaic acid (Malassezia metabolite) → tyrosinase inhibition → hypopigmentation; enlarged melanosomes + inflammation → hyperpigmentation. Same organism, different pathways = "versicolor" (multiple colors). [cite: Bolognia Dermatology 5e Ch 77; IADVL Textbook 5e]