**Core Concept**
Intertrochanteric fractures of the femur are a type of hip fracture that occurs in the intertrochanteric region, which is the area of the femur between the greater and lesser trochanters. These fractures are common in elderly patients and can lead to various complications due to the complex anatomy and blood supply of the femoral head and neck.

**Why the Correct Answer is Right**
The most common complication following intertrochanteric fracture femur is avascular necrosis (AVN) of the femoral head. This occurs due to the disruption of the blood supply to the femoral head, which is primarily provided by the medial and lateral circumflex femoral arteries. The femoral head has a double blood supply, but the lateral circumflex femoral artery is more critical in maintaining the viability of the femoral head. Intertrochanteric fractures can disrupt this blood supply, leading to AVN in a significant percentage of cases.

**Why Each Wrong Option is Incorrect**
* **Option A:** Infection is a possible complication of intertrochanteric fractures, but it is not the most common complication. Infection can occur due to the surgical intervention or the presence of a foreign body, such as a hip replacement prosthesis.
* **Option B:** Nonunion is a possible complication of intertrochanteric fractures, but it is not as common as AVN. Nonunion can occur due to the poor blood supply to the fracture site, but it is often associated with smoking and poor bone quality.
* **Option C:** Malunion is a possible complication of intertrochanteric fractures, but it is not as common as AVN. Malunion can occur due to the improper alignment of the fracture fragments, but it is often associated with inadequate fixation or poor patient compliance.

**Clinical Pearl / High-Yield Fact**
The risk of avascular necrosis of the femoral head following intertrochanteric fracture femur is higher in patients with a pre-existing femoral head lesion, such as a pre-existing AVN or a femoral neck fracture. This is because the pre-existing lesion can compromise the blood supply to the femoral head, making it more susceptible to further damage from the intertrochanteric fracture.

**Correct Answer: A. Avascular necrosis of the femoral head**