**Core Concept**
Tarsal tunnel syndrome (TTS) is a compressive neuropathy of the posterior tibial nerve as it passes through the tarsal tunnel at the ankle. This condition is often caused by inflammation or swelling within the tunnel, which can lead to pain, numbness, and weakness in the foot.

**Why the Correct Answer is Right**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder that can cause inflammation and swelling in various joints, including the ankle. In RA, the body's immune system mistakenly attacks the lining of the joints (synovium), leading to inflammation, pain, and stiffness. This inflammation can compress the posterior tibial nerve, causing TTS. The synovial inflammation in RA can also lead to tenosynovitis, which is the inflammation of the tendons and their synovial sheaths, further contributing to the compression of the nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** Ankylosing spondylitis is a type of spondyloarthritis that primarily affects the spine and sacroiliac joints. While it can cause inflammation and stiffness in the lower back and hips, it is less likely to cause TTS due to its primary involvement of the axial skeleton.

**Option B:** Osteoarthritis (OA) is a degenerative joint disease that primarily affects the cartilage, leading to joint pain and stiffness. While OA can cause inflammation and swelling in the affected joints, it is less likely to cause TTS due to its primary involvement of the joint cartilage rather than the synovium.

**Option D:** Psoriatic arthritis (PsA) is a type of spondyloarthritis that is associated with psoriasis. While PsA can cause inflammation and swelling in various joints, it is less likely to cause TTS due to its primary involvement of the distal interphalangeal joints rather than the ankle.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the underlying cause of TTS, as treatment should focus on addressing the underlying condition, such as RA, rather than just managing the symptoms. In RA, early recognition and treatment of TTS can help prevent further nerve damage and improve outcomes.

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