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? Lo...

To add to Dr. @Dr. First Last'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 conjuncti...