HLA-B27 is associated with:
**Core Concept**
HLA-B27 is a human leukocyte antigen (HLA) subtype that plays a crucial role in the immune system. It is a part of the major histocompatibility complex (MHC) class I, which presents peptide fragments from inside the cell to the immune system.
**Why the Correct Answer is Right**
HLA-B27 is associated with an increased risk of developing several autoimmune and inflammatory conditions, including ankylosing spondylitis, reactive arthritis, and psoriatic arthritis. This is because HLA-B27 has a propensity to form aggregates and activate the innate immune system, leading to chronic inflammation and tissue damage. The exact mechanism is not fully understood, but it is thought to involve the presentation of peptides from gut bacteria to HLA-B27, leading to an abnormal immune response.
**Why Each Wrong Option is Incorrect**
**Option A:** Psoriasis - While psoriasis is an autoimmune condition, HLA-B27 is not the primary genetic association. Psoriasis is more commonly associated with HLA-Cw6.
**Option B:** Rheumatoid arthritis - HLA-DRB1 is the primary genetic association with rheumatoid arthritis, not HLA-B27.
**Option C:** Multiple sclerosis - HLA-DRB1 and HLA-A2 are associated with multiple sclerosis, but not HLA-B27.
**Clinical Pearl / High-Yield Fact**
Remember that HLA-B27 is a "red flag" for ankylosing spondylitis, especially in young men, and should prompt further investigation and screening for other autoimmune conditions.
**Correct Answer: D. Ankylosing spondylitis**