**Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive wear and tear of articular cartilage, leading to joint pain, stiffness, and limited mobility. Heberden nodes are a classic clinical feature of OA, particularly affecting the distal interphalangeal joints of the fingers.

**Why the Correct Answer is Right**
Heberden nodes are bony enlargements of the distal interphalangeal joints, resulting from the deposition of calcium pyrophosphate dihydrate (CPPD) crystals or osteophyte formation. This occurs due to the abnormal stress and strain on the joint, leading to cartilage degeneration and subsequent bone changes. The formation of Heberden nodes is a hallmark of OA, often associated with symptoms such as pain, stiffness, and limited finger range of motion.

**Why Each Wrong Option is Incorrect**
**Option B:** Increased ESR (Erythrocyte Sedimentation Rate) is a non-specific marker of inflammation and is not a feature of osteoarthritis. While OA can cause joint inflammation, ESR is not a reliable indicator of the disease.

**Option C:** Onycholysis refers to the separation of the nail plate from the nail bed, commonly seen in conditions such as psoriasis or fungal infections. It is not a feature of osteoarthritis.

**Option D:** Z deformity is a characteristic of rheumatoid arthritis, where the metacarpophalangeal (MCP) joints become flexed, resulting in a "Z" shape. This is not a feature of osteoarthritis.

**Clinical Pearl / High-Yield Fact**
OA is the most common type of arthritis, affecting over 30 million people worldwide. Recognizing Heberden nodes is crucial for diagnosing OA, as it allows clinicians to initiate appropriate management strategies, such as joint protection, physical therapy, and pain relief.

**✓ Correct Answer: A. Heberden node**