**Core Concept**
Psoriatic arthritis (PsA), rheumatoid arthritis (RA), and ankylosing spondylitis (AS) are all forms of inflammatory arthritis, but they have distinct pathophysiologies and clinical features. PsA is characterized by a combination of psoriasis and arthritis, while RA is an autoimmune disease targeting the synovium. AS is a form of spondyloarthritis primarily affecting the spine and sacroiliac joints.

**Why the Correct Answer is Right**
The key to differentiating these conditions lies in their clinical and radiological features. PsA often presents with dactylitis (sausage-like swelling of the fingers or toes), enthesitis (inflammation of the sites where tendons and ligaments insert into bone), and psoriasis. RA, on the other hand, typically presents with symmetrical polyarthritis and systemic symptoms like fever and fatigue. AS is characterized by chronic back pain and stiffness, with radiographic evidence of sacroiliitis and spinal fusion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it's a general statement and doesn't specifically address the differentiating features between PsA, RA, and AS.
**Option B:** While dactylitis is a feature of PsA, it's not the only differentiating feature between these conditions.
**Option C:** Systemic symptoms like fever and fatigue are more commonly associated with RA, but are not unique to this condition.
**Option D:** This option might be correct, but it's too broad and doesn't specifically address the differentiating features between PsA, RA, and AS.

**Clinical Pearl / High-Yield Fact**
Remember the "DASH" mnemonic to help differentiate between these conditions: D (dactylitis in PsA), A (ankylosing spondylitis affecting the spine), S (sacroiliitis in AS), and H (hand involvement in RA).

**Correct Answer: C. Chronic back pain and stiffness, with radiographic evidence of sacroiliitis and spinal fusion.**