**Core Concept**
Heberden's nodes are a characteristic manifestation of osteoarthritis, specifically involving the distal interphalangeal joints of the fingers. These bony enlargements are a result of the degenerative changes in the joint, leading to the formation of hard, bony spurs.

**Why the Correct Answer is Right**
Heberden's nodes are a classic sign of osteoarthritis, particularly affecting the distal interphalangeal joints. This is due to the degeneration of the cartilage in these joints, which leads to the formation of bony spurs and subsequent joint deformity. The distal interphalangeal joints are more prone to osteoarthritis due to their anatomy, which involves a smaller joint surface area and a higher concentration of weight-bearing forces.

**Why Each Wrong Option is Incorrect**
**Option A:** The proximal interphalangeal joints (PIP) are also commonly affected by osteoarthritis, but the specific sign associated with these joints is Bouchard's nodes, not Heberden's nodes.
**Option B:** Metacarpophalangeal joints (MCP) are primarily involved in osteoarthritis in conditions such as rheumatoid arthritis, rather than osteoarthritis.
**Option C:** Metatarsophalangeal joints (MTP) are a common site of osteoarthritis, particularly in conditions such as hallux rigidus, but are not associated with Heberden's nodes.

**Clinical Pearl / High-Yield Fact**
Heberden's nodes are often bilateral and symmetrical, and their presence can be an important diagnostic clue for osteoarthritis. It's worth remembering that Bouchard's nodes are associated with the proximal interphalangeal joints, while Heberden's nodes are characteristic of the distal interphalangeal joints.

**✓ Correct Answer: A. Distal interphalangeal joint**