A 16 year-old boy presented with asymptomatic, multiple, erythematous, annular lesions with a collarette of scales at the periphery of the lesions present on the trunk. The most likely diagnosis is:
First, I need to recall the differential diagnoses for annular lesions. Tinea versicolor comes to mind because it typically presents with scaly, round or annular lesions, often on the trunk. The collarette of scales is a classic sign of tinea versicolor caused by Malassezia species. It's usually asymptomatic or mildly pruritic, which matches the asymptomatic description here.
Other possibilities include pityriasis rosea, which can have a herald patch and a collarette, but it usually starts with a single patch followed by a rash, and is more common in older children and adults. Psoriasis might have scaly plaques, but they are not typically annular. Eczema or contact dermatitis can be itchy and have various shapes, but the annular pattern isn't a hallmark.
Looking at the options (even though they aren't listed), the correct answer is likely tinea versicolor. The key points are the annular lesions with a collarette, asymptomatic, and location on the trunk. The collarette of scales is a high-yield fact for tinea versicolor. Also, Malassezia is the causative organism, and a potassium hydroxide (KOH) prep would confirm the diagnosis.
**Core Concept**
The clinical presentation of annular erythematous lesions with a collarette of scales on the trunk is classic for *tinea versicolor*, a superficial fungal infection caused by *Malassezia* species. This condition is characterized by asymptomatic or mildly pruritic, scaly, round lesions with a distinctive peripheral scale.
**Why the Correct Answer is Right**
Tinea versicolor (pityriasis versicolor) is caused by *Malassezia* yeast overgrowth on the skin. The collarette of scales at the periphery of annular lesions is a hallmark finding. The lesions occur due to fungal colonization of the stratum corneum, leading to altered keratinization and pigmentation. It is more common in adolescents and young adults, aligning with the patient’s age. The asymptomatic nature and trunk localization further support this diagnosis.
**Why Each Wrong Option is Incorrect**
**Option A:** Pityriasis rosea typically presents with a "herald patch" followed by a Christmas tree distribution of lesions, often with central clearing. It is not characterized by a collarette of scales.
**Option B:** Psoriasis may present with scaly plaques but lacks annular morphology and collarettes. Lesions are typically erythematous, well-demarcated, and silvery-scaled.
**Option C:** Eczema (atopic or contact) usually involves pruritic, inflamed, and eczematous lesions without annular patterns or collarettes.
**Clinical Pearl / High-Yield Fact**
The collarette of scales in annular lesions is a **hallmark of tinea versicolor**. A potassium hydroxide (KOH) preparation of scrapings reveals *Malassezia* hyphae and spores. Treatment