Reactive arthritis is commonly associated with infection by which organism?

• A. Chlamydia trachomatis
• B. Streptococcus pyogenes
• C. Staphylococcus aureus
• D. Escherichia coli

Answer: (A)

Reactive arthritis is an immune-driven, sterile joint inflammation that follows an extra-articular infection. The most classic trigger is Chlamydia trachomatis, typically after a genitourinary infection, though certain gut infections can also precipitate it. The arthritis itself isn’t caused by live bacteria in the joint; instead, immune responses to the initial infection drive the inflammation, which is why it can occur even after the initial infection has resolved.

Chlamydia trachomatis fits this pattern best, making it the most likely associated organism on exams. The other organisms listed aren’t the classic triggers: Streptococcus pyogenes is more associated with rheumatic fever; Staphylococcus aureus commonly causes septic arthritis rather than a reactive pattern; Escherichia coli is a frequent cause of UTIs or gastroenteritis but is not the organism most characteristically linked to reactive arthritis.