Joint Pain and Circular Rash

🗒️ Question

A 42 year old woman presents to her GP with a three week history of fatigue and joint pain. She reports a circular rash on her upper thigh that appeared after a walking trip in a forested area.

On examination, the rash is annular with central clearing and measures 8 cm in diameter. She has mild tenderness in her knees, however no swelling or redness is noted.

Investigations

• CRP: 18 mg/L (<5)

• WBC: 9 ×10⁹/L (4–11)

What is the most important investigation?

(A) Blood cultures

(B) ELISA for Borrelia antibodies

(C) MRI of the affected joints

(D) Skin biopsy of the rash

(E) Synovial fluid analysis