How do you approach evaluation and management of a patient with recurrent monoarticular inflammatory arthritis of unclear etiology?   

Routine testing will not identify Lyme disease although the WBC count is higher than usual. If the antibiotic does not target B burgdorferi recurrence might mean inappropriate therapy previously. WBC count WAAAY too high for mere trauma or internal derangement. Anything else to suggest psoriasis? ...

To add to Dr. @Leonard H. Sigal's excellent response (and on the lines of thinking about PsA), reactive arthritis can notoriously cause a "pseudoseptic" joint, especially of the knee. I'd revisit a history of possible infections (GI/genitourinary), inflammatory lower back pain, reexamine the co...