30th April 2024

A 32-year-old woman, an avid hiker, presents with a 3-week history of fatigue, headache, and joint pains.

A 32-year-old woman, an avid hiker, presents with a 3-week history of fatigue, headache, and joint pains. She reports noticing a red rash on her thigh a week after a hiking trip, which has expanded outward, forming a clear area in the centre. She recalls removing a tick from her skin after the trip. She has no significant past medical history.

Investigations:

Temperature: 37.2°C

Physical examination: A 5 cm erythematous rash with central clearing on the right thigh, no neurological deficits

Which investigation is most appropriate for confirming the diagnosis?