**Core Concept**
The extra-capsular fracture of the neck of femur involves a break in the bone outside of the joint capsule, leading to instability and potential complications. This type of fracture is common in elderly patients with osteoporotic bone.

**Why the Correct Answer is Right**
The most common complication of extra-capsular fracture of the neck of femur is avascular necrosis (AVN) of the femoral head. This occurs due to disruption of the blood supply to the femoral head, which is mainly provided by the retinacular vessels and the ligamentum teres. The retinacular vessels are often damaged in extra-capsular fractures, leading to ischemia and eventual AVN. The femoral head may also undergo collapse, resulting in severe pain and limited mobility.

**Why Each Wrong Option is Incorrect**
* **Option A:** Non-union is a possible complication of femoral neck fractures, but it is more common in intra-capsular fractures.
* **Option B:** Malunion is not a typical complication of extra-capsular fractures, as the fracture fragments are often well-aligned during surgical fixation.
* **Option D:** Osteoarthritis is a potential long-term consequence of femoral neck fractures, but it is not the most common complication of extra-capsular fractures.

**Clinical Pearl / High-Yield Fact**
To remember the risk of AVN in extra-capsular fractures, recall that the "Hippocratic" (or "Hippocratic Oath") sign is often seen on X-rays: the "H" shape formed by the fracture fragments and the preserved ligamentum teres. This sign indicates a high risk of AVN.

**Correct Answer: D. Osteoarthritis**