Reactive ahritis is caused by:
**Reactive Arthritis: Core Concept**
Reactive arthritis is a type of spondyloarthritis that occurs as a reaction to an infection, typically in the gastrointestinal or genitourinary tract. It is characterized by a combination of joint inflammation, urethritis, and conjunctivitis.
**Why the Correct Answer is Right**
The exact mechanism of reactive arthritis is not fully understood, but it is believed to be triggered by an abnormal immune response to the infection. The HLA-B27 gene is a major genetic risk factor for reactive arthritis, and individuals with this gene are more likely to develop the condition. The condition is thought to result from the abnormal activation of T cells and the production of pro-inflammatory cytokines, leading to joint inflammation and other symptoms.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because reactive arthritis is not primarily caused by an autoimmune response to self-antigens.
**Option B:** This option is incorrect because reactive arthritis is not a type of autoimmune arthritis, but rather a reactive process to an infection.
**Option C:** This option is incorrect because reactive arthritis is not typically caused by a direct viral or bacterial infection of the joints.
**Clinical Pearl / High-Yield Fact**
Reactive arthritis often occurs in individuals with a history of gastrointestinal or genitourinary infections, such as Salmonella, Shigella, or Chlamydia. The condition is often associated with the HLA-B27 gene and can be triggered by an abnormal immune response to an infection.
**Correct Answer: D. Infection in the gastrointestinal or genitourinary tract.**