**Core Concept**
HLA B27 is a specific allele of the human leukocyte antigen (HLA) complex, which plays a crucial role in the immune system. The HLA complex is responsible for presenting peptides from pathogens to T-cells, thereby initiating an immune response. An association between HLA B27 and certain autoimmune and inflammatory disorders has been established.

**Why the Correct Answer is Right**
HLA B27 is maximally associated with ankylosing spondylitis (AS), a chronic inflammatory disease primarily affecting the spine and sacroiliac joints. The presence of HLA B27 increases the risk of developing AS, particularly in individuals with a family history of the disease. The exact mechanism behind this association is not fully understood, but it is thought to involve the abnormal presentation of peptides by HLA B27 to T-cells, leading to an inappropriate immune response against self-antigens.

**Why Each Wrong Option is Incorrect**
* **Option A:** HLA B27 is not maximally associated with rheumatoid arthritis (RA), although it can be found in some patients with RA. The primary genetic risk factor for RA is the shared epitope, which is not related to HLA B27.
* **Option B:** HLA B27 is not maximally associated with psoriatic arthritis (PsA), although there is some overlap between the two conditions. PsA is more commonly associated with HLA B27 and other genetic factors.
* **Option C:** HLA B27 is not maximally associated with reactive arthritis (ReA), although it can be found in some patients with ReA. ReA is more commonly associated with HLA B27 and other genetic factors, as well as with infections such as Chlamydia trachomatis.

**Clinical Pearl / High-Yield Fact**
A patient with ankylosing spondylitis and HLA B27 positivity has a higher risk of developing uveitis, a type of eye inflammation that can cause vision loss if left untreated. Regular eye exams are essential for patients with AS and HLA B27 positivity.

**Correct Answer:** C. Ankylosing Spondylitis