[Borrelia as a differential diagnosis to monoarthritis].

In this case report, a young woman presented with recurring knee monoarthritis. Arthroscopy revealed synovitis, and histology showed chronic plasma cell-dominated inflammation. A positive serum-borrelia IgG titre supported the suspicion of Lyme arthritis despite a negative specific Borrelia PCR result on a paraffin-embedded synovial biopsy. Treatment with phenoxymethylpenicillin, initiated more than two years after symptom onset, resulted in rapid and near-complete remission. The case underscores the diagnostic challenges of Lyme arthritis and the importance of considering the diagnosis in mono- or oligoarthritis.