**Core Concept**
HLA B27 is a genetic marker associated with several autoimmune and inflammatory conditions. It is a class I major histocompatibility complex (MHC) protein that plays a crucial role in the immune system's ability to distinguish between self and non-self antigens. HLA B27 is strongly linked to a group of conditions known as spondyloarthropathies.

**Why the Correct Answer is Right**
Rheumatoid arthritis (RA) is a distinct autoimmune disease characterized by inflammation of the synovium in the joints. It is primarily associated with HLA-DRB1 and HLA-DRB4, but not with HLA-B27. The pathophysiology of RA involves a complex interplay between genetic predisposition, environmental triggers, and immune system dysregulation. In contrast, HLA-B27 is a key risk factor for ankylosing spondylitis, Reiter's syndrome (now known as reactive arthritis), and psoriatic arthritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ankylosing spondylitis is a classic example of a condition associated with HLA-B27, with a strong genetic link between the two. The majority of patients with ankylosing spondylitis are HLA-B27 positive.
**Option B:** Reiter's syndrome, or reactive arthritis, is also closely linked to HLA-B27. The genetic marker is present in a significant proportion of patients with this condition.
**Option D:** Psoriatic arthritis is another condition associated with HLA-B27, although the link is not as strong as it is with ankylosing spondylitis.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a spondyloarthropathy, remember that the presence of HLA-B27 can increase the likelihood of ankylosing spondylitis or other related conditions. However, a negative HLA-B27 result does not rule out these conditions.

**Correct Answer:**
✓ Correct Answer: C. Rheumatoid arthritis