Ddx, most likely
     Flare-up of osteoarthritis
     Crystal arthritis
          Gout
          Pseudogout
     Bacterial arthritis
          Gonorrheal     
          Staphylococcal
Sx
     Calor, dolor, rubor, tumor
Workup     
     Aspirate
          Cell count w/diff
               Cloudy or particulate w/>50k WBC: septic
               >=50k WBC w/PMN predominance: may be inflammatory
               Gram stain  negative result does not r/o septic
          Fluid cx  may take >= 2d
          Microscopic exam for crystals: 
               Gout: uric acid crystals
               Pseudogout
                    Calcium pyrophosphate dehydrate crystals
                    Rhomboid-shaped
                    Positively-biregringent crystals
          Hemarthosis:  suspect coagulopathy if atraumatic
          If traumatic, fat droplets from bone marrow in aspirate indicates intraarticular bony fracture

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