A 32-year-old woman presents with a scaly rash on her trunk and upper arms that started 3 weeks ago. KOH mount shows septate hyphae. Which single feature best distinguishes tinea corporis from tinea versicolor?

Explanation

## Distinguishing Tinea Corporis from Tinea Versicolor ### The Best Single Distinguishing Feature **Key Point:** The question asks which single feature **best distinguishes** tinea corporis from tinea versicolor. The correct answer is **Option C — Hypopigmented macules that do not tan in summer**, which is a hallmark clinical feature of **tinea versicolor** (pityriasis versicolor) and is absent in tinea corporis. ### Why Option C Is Correct Tinea versicolor is caused by *Malassezia furfur/globosa*, a lipophilic yeast. The organism produces **azelaic acid**, which inhibits tyrosinase in surrounding melanocytes, leading to hypopigmented patches. These areas **fail to tan with sun exposure** because melanocyte function is suppressed — a feature entirely absent in tinea corporis. This clinical sign is pathognomonic for tinea versicolor and is the single best clinical discriminator between the two conditions. | Feature | Tinea Corporis | Tinea Versicolor | |---------|---|---| | **Causative organism** | Dermatophytes (*Trichophyton*, *Microsporum*, *Epidermophyton*) | *Malassezia furfur/globosa* | | **KOH morphology** | Septate hyphae | Short hyphae + round spores ("spaghetti and meatballs") | | **Hypopigmented non-tanning macules** | ❌ Absent | ✅ Classic/pathognomonic feature | | **Lesion morphology** | Annular plaques with active border, central clearing | Confluent hypopigmented/hyperpigmented macules on trunk | | **Pruritus** | Variable | Minimal to absent | ### Why Other Options Are Incorrect - **Option A (Pruritus):** Both tinea corporis and tinea versicolor can be pruritic or minimally symptomatic; pruritus is not a reliable discriminator between the two. - **Option B (Septate hyphae on KOH):** This is already stated in the clinical stem as a given finding. Furthermore, while septate hyphae are characteristic of dermatophytes (tinea corporis), the question asks for the *best distinguishing* feature — and a KOH finding that is already provided in the vignette cannot serve as the answer. The KOH morphology *does* differ (septate hyphae vs. "spaghetti and meatballs"), but the option as written ("presence of septate hyphae") is a finding already established in the stem, making it circular. - **Option D (Intertriginous involvement):** Both tinea corporis and tinea versicolor can involve intertriginous areas; this is not discriminatory. **Clinical Pearl:** The failure to tan in summer (despite sun exposure) in hypopigmented macules on the trunk is pathognomonic for tinea versicolor due to azelaic acid–mediated melanocyte inhibition. This feature is absent in tinea corporis, making it the single best clinical discriminator. *(Andrews' Diseases of the Skin; Fitzpatrick's Dermatology in General Medicine)* **High-Yield:** Tinea versicolor → *Malassezia* → azelaic acid → tyrosinase inhibition → hypopigmented macules that do NOT tan. Tinea corporis → dermatophytes → annular erythematous plaques with central clearing, no melanocyte involvement.