**Core Concept**
Primary osteoarthritis (OA) of the knee joint is a degenerative joint disease characterized by progressive wear and tear of the articular cartilage, leading to joint space narrowing and deformation of the surrounding bone. This condition is often associated with age-related changes, obesity, and genetic predisposition. The knee joint is a complex synovial joint that comprises the femur, tibia, and patella, with multiple ligaments and menisci providing stability.

**Why the Correct Answer is Right**
The most common deformity seen in primary osteoarthritis of the knee joint is **varus deformity**. This occurs due to the uneven wear and tear of the articular cartilage, leading to the medial (inner) compartment of the knee joint becoming more affected than the lateral (outer) compartment. As a result, the knee joint collapses inward, causing the medial aspect of the knee to become more prominent than the lateral aspect. This varus deformity is often accompanied by joint space narrowing, osteophytes formation, and pain.

**Why Each Wrong Option is Incorrect**
* **Option A:** Valgus deformity is incorrect because it is more commonly associated with traumatic injuries to the knee joint, such as fractures or ligamentous injuries, rather than primary osteoarthritis.
* **Option B:** Flexion contracture is incorrect because it is more commonly associated with conditions such as spinal cord injuries or cerebral palsy, which affect muscle tone and lead to joint contractures.
* **Option C:** Extension contracture is incorrect because it is more commonly associated with conditions such as polio or stroke, which affect muscle tone and lead to joint contractures.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that varus deformity in primary osteoarthritis of the knee joint is often associated with a positive **Lachman test**, which indicates ligamentous laxity and increased joint instability.

**Correct Answer: . Varus deformity.**