A 35-year-old man from Himachal Pradesh presents with a 10-day history of an expanding rash on his right thigh following a hiking trip. On examination, he has a well-demarcated annular lesion with central erythema, a clear ring of partial clearing, and an outer expanding red ring (marked **A** in the diagram). He is afebrile but reports mild headache and myalgia. He denies pruritus or pain at the lesion site. What is the most likely diagnosis?

Explanation

## Why Early localized Lyme disease (erythema migrans) is right The classic "bull's-eye" or target appearance marked **A** in the diagram—with central erythema, clear ring of partial clearing, and outer expanding red ring—is the **pathognomonic rash of early localized Lyme disease** caused by *Borrelia burgdorferi*. The timeline (10 days post-exposure, within the 3–30 day window with median 7–14 days), the painless and non-pruritic nature, the annular centrifugal expansion, and the associated systemic symptoms (headache, myalgia) all support this diagnosis. In endemic regions (including increasing recognition in India's hill stations), clinical diagnosis of classic erythema migrans does not require serologic confirmation—treatment should be initiated immediately. [Murray 9e; Harrison 21e Ch 181] ## Why each distractor is wrong - **Tinea corporis with secondary bacterial infection**: Fungal infections typically present with pruritus and scale; the rash is usually unilateral and does not expand as rapidly or symmetrically. The painless, non-pruritic nature and the precise target morphology are not typical of dermatophyte infections. - **Contact dermatitis from plant exposure**: Contact dermatitis is typically pruritic and painful, not painless. The morphology is usually less well-demarcated and lacks the characteristic annular centrifugal expansion with a clear central ring seen in erythema migrans. - **Erythema multiforme triggered by an occult viral infection**: Erythema multiforme presents with fixed, painful lesions (often on palms and soles) with a true "target" of three zones (central blister, pale zone, outer erythema). It does not expand centrifugally and is associated with prior infection or drug exposure, not a recent tick bite. **High-Yield:** Erythema migrans = painless, non-pruritic, expanding annular rash with bull's-eye appearance at tick bite site, 3–30 days post-bite; clinical diagnosis in endemic regions; treat with doxycycline 100 mg BID × 14–21 days. [cite: Murray 9e; Harrison 21e Ch 181]