Reactive arthritis (formerly known as Reiter’s syndrome) is an inflammatory arthritis that develops as an autoimmune response triggered by a bacterial infection elsewhere in the body, most often in the genitourinary tract (commonly Chlamydia trachomatis) or gastrointestinal tract (such as Salmonella, Shigella, Campylobacter, or Yersinia), typically 1–4 weeks after the initial infection. It primarily affects joints (causing painful swelling, often asymmetric in lower limbs like knees, ankles, or feet), eyes (conjunctivitis or uveitis leading to redness, pain, or light sensitivity), and urethra (urethritis with burning urination or discharge). Additional common features include skin manifestations (e.g., keratoderma blennorrhagicum on palms/soles or circinate balanitis on the glans penis), mouth ulcers, enthesitis (inflammation at tendon insertions, such as Achilles tendon pain), dactylitis (sausage-like swelling of fingers or toes), and occasionally low back pain from sacroiliitis. It most frequently occurs in young adults, with a higher incidence in men, and while the triggering infection may have cleared by symptom onset, the sterile joint inflammation persists as an immune-mediated process.