**Core Concept**
Primary osteoarthritis is a degenerative joint disease characterized by progressive wear and tear of the articular cartilage, leading to joint pain, stiffness, and limited mobility. The condition is often associated with age-related changes, genetics, and mechanical stress on the joints.

**Why the Correct Answer is Right**
The correct answer is determined by the joints' susceptibility to primary osteoarthritis. Joints with a higher risk of primary osteoarthritis have limited synovial fluid, lower articular cartilage thickness, and a higher incidence of joint degeneration. The metacarpophalangeal (MCP) joints, particularly the first carpometacarpal joint (CMC) of the thumb, are commonly affected due to their high mechanical stress and limited synovial fluid. In contrast, the first metatarsophalangeal joint (MTP) is relatively less involved in primary osteoarthritis.

**Why Each Wrong Option is Incorrect**
* **Option A:** The distal interphalangeal (DIP) joints are commonly involved in primary osteoarthritis, especially in the fingers.
* **Option B:** The proximal interphalangeal (PIP) joints are also frequently affected by primary osteoarthritis, often resulting in Heberden's nodes.
* **Option C:** The hip joint is a common site for primary osteoarthritis, particularly in older adults, due to its high mechanical stress and limited synovial fluid.

**Clinical Pearl / High-Yield Fact**
The first carpometacarpal joint (CMC) of the thumb is the most commonly affected joint in primary osteoarthritis, often leading to thumb pain and limited mobility.

**Correct Answer: D. First metatarsophalangeal (MTP) joint.**