Reactive arthritis (Reiter's syndrome) is most commonly triggered by which of the following?

• A. Chlamydia or bacterial GI infection causing triad of mono-arthritis, conjunctivitis, and urethritis, sometimes with sacroiliitis.
• B. Viral upper respiratory infection.
• C. Autoimmune reaction to dairy.
• D. Fungal skin infection.

Answer: (A)

Reactive arthritis is an autoimmune inflammatory response that follows an infection outside the joint, not an infection inside the joint itself. The classic triggers are genitourinary infections with Chlamydia trachomatis or certain bacterial gastroenteritis pathogens (such as Shigella, Salmonella, Campylobacter, or Yersinia). After the triggering infection, the body mounts an inflammatory reaction in the joints, often presenting as an asymmetric arthritis, along with urethritis and conjunctivitis—the triad that points to this post-infectious process. This is why the option linking a Chlamydia or bacterial GI infection to arthritis plus urethritis and conjunctivitis is the best choice. Viral upper respiratory infections can be associated in some cases but are not the most common triggers, and dairy or fungal skin infections do not trigger this syndrome.