**Core Concept**
Psoriatic arthritis (PsA) is a chronic inflammatory condition characterized by the association of psoriasis with joint inflammation and damage. The pathophysiology of PsA involves an abnormal immune response, with T cells and other inflammatory cells contributing to joint damage.

**Why the Correct Answer is Right**
The most common form of psoriatic arthritis is peripheral arthritis, which presents as oligoarticular or polyarticular arthritis, typically affecting the distal interphalangeal joints and proximal interphalangeal joints. This form of PsA is often associated with psoriasis involving the skin, and the joint involvement can be symmetrical or asymmetrical. The exact mechanism of PsA is not fully understood, but it is believed to involve an interplay between genetic predisposition, environmental factors, and an abnormal immune response.

**Why Each Wrong Option is Incorrect**
* **Option A:** Axial PsA, which affects the spine and sacroiliac joints, is a less common form of PsA, accounting for approximately 10-20% of cases.
* **Option B:** Dactylitis, or "sausage digit," is a characteristic feature of PsA, but it is not the most common form of the disease. Dactylitis involves inflammation of an entire digit, leading to swelling and pain.
* **Option D:** Psoriatic spondylitis is a rare form of PsA that primarily affects the spine and pelvis, often presenting with symptoms similar to those of ankylosing spondylitis.

**Clinical Pearl / High-Yield Fact**
PsA can be challenging to diagnose, as the symptoms can be similar to those of other forms of arthritis. A thorough medical history, physical examination, and laboratory tests, including radiographic imaging, are essential for accurate diagnosis.

**Correct Answer: C. Peripheral arthritis**