**Core Concept**
Primary osteoarthritis is a degenerative joint disease characterized by progressive wear and tear of articular cartilage, leading to bone-on-bone contact and subsequent joint inflammation. This condition is often associated with aging, obesity, and genetic predisposition.

**Why the Correct Answer is Right**
Primary osteoarthritis primarily affects weight-bearing joints, such as the hips and knees, as well as joints that are subjected to repetitive stress and strain, such as the hands and spine. The condition arises from a combination of mechanical, biochemical, and molecular factors, including the loss of proteoglycans and collagen in the cartilage matrix, leading to decreased cartilage thickness and increased bone density. The subchondral bone undergoes sclerosis and cyst formation due to the altered mechanical loading.

**Why Each Wrong Option is Incorrect**
**Option A:** Not provided. (Assuming this is the correct option, but the question is incomplete.)
**Option B:** This is incorrect as primary osteoarthritis can affect the hands, particularly the distal interphalangeal joints, due to repetitive strain and genetic predisposition.
**Option C:** This is incorrect as primary osteoarthritis can affect the spine, particularly the lumbar and cervical regions, due to degenerative changes and mechanical stress.
**Option D:** This is incorrect as primary osteoarthritis can affect the hips and knees, particularly in individuals with a genetic predisposition or obesity.

**Clinical Pearl / High-Yield Fact**
Primary osteoarthritis often presents with pain and stiffness in the affected joints, particularly after periods of inactivity, and is often accompanied by crepitus and limited range of motion.

**Correct Answer:** A.