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Lyme disease — MCCQE Part 1 MCQ

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EasyRashLyme diseaseMCCQE Part 1

Ten days after removing an attached blacklegged tick while camping in eastern Ontario, a 27-year-old develops an expanding 14-cm erythematous patch at the bite site. It is warm but not painful or pruritic and has partial central clearing. There is no fluctuance or purulent drainage. What is the best next step?

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Correct answer: E — Treat clinically with oral doxycycline without waiting for Lyme serology

A progressively expanding lesion larger than 5 cm after credible blacklegged-tick exposure is erythema migrans, the characteristic presentation of early localized Lyme disease. It is a clinical diagnosis, so an appropriate oral regimen such as doxycycline should start promptly. Antibodies may not yet be detectable during early localized infection; a negative result could therefore falsely reassure, and testing should not delay treatment of a classic lesion. The size, expansion and absence of a scaly raised border make tinea unlikely. Lack of tenderness, diffuse painful erythema or purulence argues against routine cellulitis treatment. Observation risks neurologic, cardiac or joint dissemination. Drug choice is individualized for pregnancy, age, allergy and other contraindications, but the management principle remains immediate clinical treatment.

Reference: PHAC Lyme Disease Guidance for Health Professionals: https://www.canada.ca/en/public-health/services/diseases/lyme-disease/health-professionals-lyme-disease.html