Monoarthropathies are common and carry a broad differential including infection, trauma, lupus, rheumatoid arthritis, or crystal arthropathies.1–3 Of these, the can’t miss diagnosis is septic arthritis, as delayed treatment can cause irreversible joint destruction with a fatality rate up to 11% if not treated.4  Yikes. The Patient You are working in minor treatment and a 70-year-old gentleman presents with pain in his left knee that has been worsening over the past week.  It …