## **Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder that primarily affects joints, and its pathogenesis involves a combination of genetic and environmental factors. The human leukocyte antigen (HLA) system plays a significant role in the genetic predisposition to RA. Specific HLA alleles are associated with an increased risk of developing RA.

## **Why the Correct Answer is Right**
The correct answer, **HLA-DRB1**, is associated with rheumatoid arthritis, particularly with the shared epitope hypothesis. This hypothesis suggests that certain alleles of the *HLA-DRB1* gene, such as *HLA-DRB1*0401 and *HLA-DRB1*0404, share a common amino acid sequence in their third hypervariable region. This sequence is thought to contribute to the abnormal antigen presentation and subsequent autoimmune response characteristic of RA. The presence of these alleles increases the risk and severity of RA.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *HLA-A* is not specifically associated with RA in the same way *HLA-DRB1* is. While *HLA-A* alleles may influence susceptibility to certain autoimmune diseases, they are not primarily linked to RA.
- **Option B:** *HLA-C* also does not have a well-established association with RA comparable to *HLA-DRB1*. Some *HLA-C* alleles may influence disease susceptibility in other autoimmune conditions but are not key factors in RA.
- **Option D:** *HLA-DQ* has associations with other autoimmune diseases, such as type 1 diabetes and celiac disease, but it is not primarily linked to RA in the same manner as *HLA-DRB1*.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the *HLA-DRB1* shared epitope alleles not only increase the risk of developing RA but also correlate with disease severity and the presence of rheumatoid factor. This association can help in understanding the potential disease course and guiding management strategies.

## **Correct Answer:** . **HLA-DRB1**