Lyme Disease — SCE Dermatology MCQ

Correct answer: C — Erythema migrans (Lyme disease) — Doxycycline 100 mg BD for 21 days

The correct answer is C, erythema migrans (Lyme disease), treated with doxycycline 100 mg BD for 21 days. The stem describes a tick bite in an endemic UK area (the New Forest is a recognised hotspot for Ixodes ticks and Borrelia burgdorferi) followed weeks later by a slowly expanding annular erythematous plaque with central clearing, the "bull's eye" appearance pathognomonic for erythema migrans. NICE guidance states this rash is diagnosed clinically and treatment should start without waiting for serology, since antibody tests are often negative at this early localised stage. First-line therapy without focal neurological or cardiac involvement is oral doxycycline 100 mg twice daily (or 200 mg once daily) for 21 days, with amoxicillin as the alternative if doxycycline is contraindicated (for example in pregnancy). Why the other options are wrong: B. Fixed drug eruption: produces a well-demarcated, often solitary violaceous or hyperpigmented patch that recurs at the same site on re-exposure to a specific drug, not a progressively expanding ring following a tick bite. E. Subacute cutaneous lupus: presents with photosensitive, scaly, annular or papulosquamous lesions typically on sun-exposed skin such as the upper trunk and arms, with no temporal link to a tick bite, so hydroxychloroquine is not indicated. A. Tinea corporis: causes an itchy, scaly annular plaque with an active scaling border, usually smaller and pruritic, and is unrelated to tick exposure; topical terbinafine treats fungal infection, not spirochaetal disease. D. Granuloma annulare: gives asymptomatic, non-scaly, skin-coloured to violaceous annular papules or plaques usually on the dorsal hands or feet, with no infective cause and no relation to tick bites. Key point: any expanding annular erythema with central clearing after a tick bite in an endemic UK area is erythema migrans until proven otherwise, and warrants immediate empirical doxycycline without waiting for serology.

Reference: NICE NG95, Lyme disease (2018, updated), Table 1: Antibiotic treatment, https://www.nice.org.uk/guidance/ng95