Inflammatory

• Infection, eg, Staphylococcus aureus septic arthritis, gonococcal arthritis, endocarditis, parvovirus B19
• Crystal-induced, eg, gout, pseudogout
• Reactive arthritis (Reiter's syndrome)
• Uncommon: other seronegative spondyloarthropathy, connective tissue disease

Noninflammatory

• Trauma
• Nontraumatic bleed, eg, coagulopathy
• Sickle cell crisis

• Arthritis, general DDx
• Knee effusion