**Core Concept**
Primary osteoarthrosis, also known as primary osteoarthritis, is a degenerative joint disease characterized by progressive loss of articular cartilage and underlying bone changes. The pathophysiology involves mechanical stress, inflammatory responses, and altered joint biomechanics.

**Why the Correct Answer is Right**
The most common site of primary osteoarthrosis is the weight-bearing joints, particularly the knee joint. This is due to the high frequency of mechanical stress and repetitive loading on these joints, leading to cartilage wear and tear. The knee joint has a relatively shallow joint space and a large surface area, making it more susceptible to cartilage damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Hip joint is also a common site, but it is less common than the knee joint.
**Option B:** Ankle joint is not typically considered a common site for primary osteoarthrosis.
**Option C:** Shoulder joint can be affected, but it is less common than the knee and hip joints.

**Clinical Pearl / High-Yield Fact**
Remember that the risk of developing primary osteoarthrosis increases with age, obesity, and previous joint injuries, making it essential to maintain a healthy lifestyle and manage any underlying conditions to reduce the risk of developing this condition.

**Correct Answer: C. Hip joint.**