In the absence of trauma, acute monoarthritis is most likely to be caused by septic or crystalline arthritis.
In a febrile patient with a joint effusion, diagnostic arthrocentesis is mandatory. Inflammatory fluid, that is, a white blood cell count >2000/mm-\ should be considered infected until proven otherwise.
Gonococcal arthritis usually presents as a migratory tenosynovitis, often involving the wrists and hands, with few vesicopustular skin lesions. Nongonococcal septic arthritis is most often caused by Staphylococcus aureus and most often affects large weight-bearing joints.
Monosodium urate crystals in gout are needle-shaped and negatively birefringent (yellow) under the polarizing microscope. Calcium pyrophosphate dihydrate crystals in pseudogout are rhomboid and positively birefringent (blue).
Treatment of gout depends on the stage: nonsteroidal antiinflammatory drugs, colchicine, or steroids for an acute gouty arthritis, and urate lowering with probenecid or allopurinol during the intercritical period.
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