**Question:** Pannus is seen in -

A. Osteoarthritis
B. Rheumatoid arthritis
C. Ankylosing spondylitis
D. Systemic sclerosis

**Core Concept:** Pannus is a synovial tissue mass that develops in response to chronic inflammation, leading to destruction of adjacent bone and cartilage.

**Why the Correct Answer is Right:** Pannus is a unique feature of rheumatoid arthritis (RA), a systemic autoimmune disease that primarily affects synovial joints. In RA, the immune system mistakenly attacks the synovial tissue, causing inflammation and proliferation of synovial cells. This leads to the formation of pannus, which is characterized by increased blood vessels, infiltration of inflammatory cells, and production of proteolytic enzymes. Pannus can destroy adjacent bone and cartilage, leading to joint deformity and dysfunction.

**Why Each Wrong Option is Incorrect:**

A. Osteoarthritis (OA): OA is a degenerative joint disease caused by wear and tear of articular cartilage, subchondral bone, and joint capsule. Pannus is not a feature of OA.

B. Rheumatoid arthritis (RA): As mentioned earlier, pannus is a defining feature of RA, making this the correct answer.

C. Ankylosing spondylitis (AS) is a form of inflammatory arthritis primarily affecting the spine. Pannus is not a prominent feature in AS, as it primarily involves the spine and sacroiliac joints.

D. Systemic sclerosis (scleroderma): This is a connective tissue disorder characterized by fibrosis and vasculopathy, affecting multiple organ systems but not typically involving synovial joints. Pannus is not a feature of systemic sclerosis.

**Clinical Pearl:**

Understanding the key pathological features of rheumatoid arthritis is essential for accurate diagnosis and appropriate management of patients with joint pain and inflammation. Pannus formation is a critical factor in the progression of joint damage in RA, which is why it is a defining feature of this disease.