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Acute monarthritis should be regarded as infectious until proved otherwise. Early evaluation is crucial because of the capacity of some infectious agents to destroy cartilage rapidly. The history and physical examination can provide highly suggestive clues, but a definitive diagnosis may depend on arthrocentesis and analysis of synovial fluid. The diagnosis of acute monarthritis is rarely established by radiography. The most common cause of bacterial arthritis is Neisseria gonorrhoeae. Staphylococcus aureus and streptococci are the organisms most frequently implicated in nongonococcal bacterial arthritis, although the possibility of Gram-negative bacteria or anaerobes should not be overlooked in intravenous drug users or immunocompromised patients. Inflammation in a large joint, particularly the knee, might arouse suspicion of
Lyme disease. Other, less frequently encountered infectious causes of acute monarthritis include tuberculosis and other mycobacteria, fungi, and viruses. Arthroscopic examination and synovial tissue biopsy may be necessary to diagnose such processes. Microscopic examination of the synovial fluid may reveal a crystalline etiology for monarthritis. Monosodium urate crystals induce gout, usually in the toe, ankle, or midfoot, while calcium pyrophosphate crystals cause pseudogout, most often in the knee or wrist. Acute monarthritis is sometimes a manifestation of osteoarthritis or an early sign of a systemic arthritis such as rheumatoid or reactive arthritis. Processes underlying acute monarthritis can also evolve into a more chronic clinical picture as exemplified by the spondyloarthropathies.