Monoarthritis (differential diagnosis)

• osteoarthritis - acute exacerbation
• gout - especially the great toe often affects an already arthritic joint
• septic arthritis (this is an orthopaedic emergency that can cause significant joint damage leading to increased morbidity and mortality. Early diagnosis and treatment are crucial for preserving joint function.)
• psoriatic arthropathy
• rheumatoid arthritis - acute attack
• pseudogout - especially the knee may affect an already arthritic joint
• viral infection - especially rubella and parvovirus
• blackthorn synovitis
• reactive arthritis - typically in a young man up to month after urethritis or gastroenteritis, affecting the lower limb and occurring before any eye manifestations
• trauma/haemarthrosis
• Lyme disease

Reference

1. Ma L, Cranney A, Holroyd-Leduc JM. Acute monoarthritis: what is the cause of my patient's painful swollen joint? CMAJ. 2009 Jan 06;180(1):59-65.