Probable Diagnosis: Erythema Migrans (Lyme Disease - Early Localized Stage)

Why This Fits

• Per [Andrews' Diseases of the Skin](Andrews' Diseases of the Skin, p. 334): EM begins as a red macule or papule expanding outward over days to weeks; "the advancing border is usually slightly raised, warm, red to bluish-red, and free of any scale." The median diameter reaches ~15 cm (range 3-68 cm).
• The lesion is typically painless and nonpruritic (which differentiates it from contact dermatitis). About 50% have a burning sensation.
• Per the [Red Book 2021](Red Book 2021, p. 774): EM is diagnosed clinically based on appearance in the correct seasonal and geographic context. No serology is needed if EM is present - it's the most characteristic manifestation.

Differential Diagnoses to Consider

1. Tinea corporis - would show scaling along the advancing border (absent here); KOH prep positive
2. Erythema Annulare Centrifugum (EAC) - slower expanding, often has a trailing inner scale
3. Southern Tick-Associated Rash Illness (STARI) - if in southern US states, caused by Amblyomma americanum tick; clinically identical to EM, etiology unknown
4. Cellulitis - does not form rings; more painful, no central clearing
5. Granuloma annulare - skin-colored papules in a ring, not erythematous; no tick bite history
6. Allergic reaction to tick bite - smaller (<5 cm), more pruritic, does not expand

Clinical Assessment

• History of tick bite (~50% of patients recall one) - 3-32 days prior (median 11 days)
• Recent outdoor/wooded area exposure (April-October season)
• Constitutional symptoms: malaise, headache, mild neck stiffness, myalgia, arthralgia, low-grade fever?
• Geographic location - endemic areas include northeastern/mid-Atlantic US, upper Midwest, Europe, Asia

Treatment

First-Line (Early Localized/Uncomplicated EM)

• Doxycycline is preferred in adults - it also covers co-infections like Anaplasma
• A 10-day course of doxycycline is accepted in some guidelines if EM is the only manifestation
• Do not use macrolides (azithromycin, erythromycin) - less effective, higher relapse rate

If Early Disseminated Disease is Present (multiple EM lesions, Bell's palsy, meningitis, carditis):

• Extend oral antibiotics to 21 days, or use IV ceftriaxone 2g/day x 14-21 days for neurological or severe cardiac involvement

Adjunctive

• NSAIDs for constitutional symptoms (arthralgia, myalgia, headache)
• Monitor for treatment response: EM should begin fading within days of antibiotic initiation
• If fever persists >24-48 hours after starting treatment, consider co-infection (babesiosis, anaplasmosis)

Important Notes

• Do not delay treatment waiting for serology - treat clinically if EM is present
• Follow-up at 2-4 weeks to confirm resolution
• If lesion expands or new lesions appear, or neurological/cardiac symptoms develop, reassess for disseminated disease
• A formal in-person evaluation by a clinician is necessary for definitive diagnosis and prescription

Disclaimer: This analysis is based on image appearance and published clinical criteria. It is not a substitute for in-person clinical evaluation. A qualified healthcare provider must examine the patient and confirm the diagnosis.