**Core Concept**
Rheumatoid arthritis (RA) and ankylosing spondylitis (AS) are two distinct autoimmune inflammatory diseases affecting the joints. While both conditions share some common pathophysiological features, they have distinct clinical manifestations and radiological characteristics.

**Why the Correct Answer is Right**
In RA, systemic inflammation is characterized by the presence of **rheumatoid factor (RF)**, an autoantibody directed against the Fc portion of IgG. RF is seen in approximately 70-80% of patients with RA, but its presence is less common in AS, where it is found in only about 5-10% of patients. This difference in RF positivity reflects the distinct immune mechanisms and joint involvement in these two diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because AS is more commonly associated with the presence of **HLA-B27**, a genetic marker that is found in approximately 90% of patients with AS. While some patients with RA may also carry HLA-B27, its presence is not as strongly associated with the disease as it is with AS.
**Option B:** This option is incorrect because both RA and AS can present with **sacroiliitis**, an inflammatory condition affecting the sacroiliac joints. However, sacroiliitis is more commonly seen in AS, where it is a hallmark feature of the disease.
**Option C:** This option is incorrect because both RA and AS can involve the **small joints of the hands and feet**, but the pattern of joint involvement differs between the two diseases. RA typically involves the small joints of the hands and feet symmetrically, whereas AS tends to involve the axial skeleton and larger joints.
**Option D:** This option is incorrect because both RA and AS can be associated with **extra-articular manifestations**, such as eye inflammation and lung disease. However, the specific extra-articular manifestations and their frequency differ between the two diseases.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with inflammatory back pain, consider the presence of RF and HLA-B27 to help differentiate between RA and AS. RF positivity is more common in RA, while HLA-B27 positivity is more characteristic of AS.

**Correct Answer: B. Sacroiliitis**