**Core Concept**
Primary osteoarthritis of the knee joint is a degenerative condition characterized by progressive wear and tear of the articular cartilage, leading to joint deformity. This condition often affects weight-bearing joints, such as the knees, hips, and spine.

**Why the Correct Answer is Right**
In primary osteoarthritis of the knee joint, the degeneration of articular cartilage and surrounding soft tissues can cause the medial (inner) compartment of the knee to become over-loaded, resulting in varus deformity. This is because the lateral (outer) compartment, which is responsible for absorbing lateral forces, becomes relatively weaker and less able to bear weight. As a result, the knee joint begins to bow outward, resulting in genu varus. This deformity can be exacerbated by factors such as obesity, age, and pre-existing knee injuries.

**Why Each Wrong Option is Incorrect**
**Option A:** Genu valgum, also known as "knock knees," is a condition where the knees angle inwards, away from the midline of the body. This is more commonly associated with rickets or Blount's disease, rather than primary osteoarthritis.

**Option B:** Genu recurvatum is a condition where the knee joint hyper-extends, or bends backward, beyond its normal range of motion. This can be caused by trauma, muscle imbalances, or neurological disorders, but is not a typical deformity seen in primary osteoarthritis.

**Option D:** Procurvatum is not a recognized medical term, and is likely a distractor. If you're thinking of a deformity that causes the knee to bow outward, you might be thinking of genu varus, which is the correct answer.

**Clinical Pearl / High-Yield Fact**
Remember the "medial compartment overload" mechanism, which is responsible for the development of genu varus in primary osteoarthritis. This is a key concept to understand when evaluating knee deformities in patients with degenerative joint disease.

**✓ Correct Answer: C. Genu varus**