What symptom is commonly observed in reactive arthritis?

Reactive arthritis is an inflammatory joint condition that occurs in response to an infection in another part of the body, most often after a urinary or gastrointestinal infection. One of the hallmark symptoms of reactive arthritis is joint swelling, specifically in the lower extremities, which may occur days to weeks following the initial infection. This condition often includes other features such as urethritis, but it is primarily characterized by the inflammatory response leading to joint involvement.

While urethritis can indeed occur in reactive arthritis, the defining symptom that directly represents the condition is joint swelling. This swelling occurs due to the body's immune response to the triggering infection and is a key clinical finding in patients diagnosed with reactive arthritis. Thus, it is crucial to recognize that joint swelling is not only a common symptom but also a central aspect of the diagnosis and understanding of reactive arthritis.