HLA-B27 is associated with
**Core Concept**
HLA-B27 is a human leukocyte antigen (HLA) allele that plays a crucial role in the immune system. It is a class I MHC molecule that presents peptides from inside the cell to CD8+ T cells, which can trigger an immune response. HLA-B27 is associated with several autoimmune and inflammatory conditions, particularly those affecting the joints and spine.
**Why the Correct Answer is Right**
HLA-B27 is strongly associated with ankylosing spondylitis (AS), a type of inflammatory arthritis that primarily affects the spine. The exact mechanism is unclear, but it is believed that HLA-B27-presented peptides trigger an abnormal immune response, leading to inflammation and tissue damage in the spine. This process involves the activation of T cells, which release cytokines and other inflammatory mediators, causing the characteristic symptoms of AS.
**Why Each Wrong Option is Incorrect**
**Option A:** Psoriasis is a chronic skin condition characterized by thickened, scaly skin lesions, but it is not directly associated with HLA-B27. While some patients with psoriasis may also have AS, the two conditions are not linked by HLA-B27.
**Option B:** Crohn's disease is a type of inflammatory bowel disease (IBD) that affects the gastrointestinal tract, but it is not directly associated with HLA-B27. While some patients with Crohn's disease may also have AS, the two conditions are not linked by HLA-B27.
**Option C:** Rheumatoid arthritis (RA) is a type of autoimmune arthritis that primarily affects the hands and feet, but it is not directly associated with HLA-B27. While some patients with RA may also have AS, the two conditions are not linked by HLA-B27.
**Clinical Pearl / High-Yield Fact**
HLA-B27 is a useful marker for identifying patients at risk of developing AS, particularly in those with a family history of the condition. However, a positive HLA-B27 test is not diagnostic on its own, and a diagnosis of AS requires a combination of clinical symptoms, laboratory tests, and imaging studies.
**Correct Answer: D. Spondyloarthritis**