## **Core Concept**
The question assesses knowledge of complications arising from neck of femur fractures, which is a significant orthopedic injury, particularly in elderly populations. Complications can be immediate or delayed and may involve the local anatomy or be systemic. Understanding these complications is crucial for managing patients effectively.

## **Why the Correct Answer is Right**
Nonunion and avascular necrosis are well-recognized complications of femoral neck fractures due to disruption of the blood supply to the femoral head. Malunion can occur but is less common and usually not listed as a major complication compared to nonunion and avascular necrosis.

## **Why Each Wrong Option is Incorrect**
- **Option A: Nonunion** - This is a known complication of femoral neck fractures. The fracture may not heal properly, leading to ongoing pain and disability.
- **Option B: Avascular necrosis** - This occurs due to the interruption of the blood supply to the femoral head, leading to necrosis of the bone tissue.
- **Option C: Malunion** - While possible, it's less commonly emphasized as a major complication compared to nonunion and avascular necrosis but still represents a potential outcome.
- **Option D: Deep vein thrombosis** - This is indeed a complication but is more of a systemic complication related to immobilization rather than a direct local complication of the fracture itself like nonunion or avascular necrosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of avascular necrosis increases with the degree of fracture displacement and the timing of surgical intervention. Early diagnosis and treatment are crucial in minimizing complications.

## **Correct Answer:** D. Deep vein thrombosis