**Core Concept:** Fracture neck of femur is a type of hip fracture, often seen in elderly patients, which can lead to non-union if not properly managed. Non-union refers to a fracture that fails to heal over time, potentially causing significant morbidity and disability.

**Why the Correct Answer is Right:** Non-union of a fracture neck of femur may occur due to a combination of factors, including poor blood supply to the fracture site, immobilization, delayed presentation, and inadequate surgical management. Inadequate blood supply to the fracture site is a crucial factor that contributes to non-union. The blood supply in the neck of the femur is provided by branches of the external iliac artery and vein, which can be compromised by factors like atherosclerosis, embolism, or thrombosis.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Atherosclerosis):** While atherosclerosis can cause compromised blood supply, it is not the only cause of non-union in fracture neck of femur. Other factors like immobilization, delayed presentation, and inadequate surgical management can also contribute to non-union.

B. **Option B (Immobilization):** While immobilization can delay healing, it is not the sole cause of non-union in fracture neck of femur. Compromised blood supply is also essential for healing, as mentioned above.

C. **Option C (Atherosclerosis and Immobilization):** Although both atherosclerosis and immobilization can contribute to non-union, they are not the only factors involved. Inadequate surgical management and compromised blood supply are also essential components for non-union.

D. **Option D (Inadequate Surgery):** While inadequate surgical management can lead to non-union, other factors like compromised blood supply and immobilization are also essential for the occurrence of non-union.

**Clinical Pearl:** The correct answer (Atherosclerosis, Immobilization, Inadequate Surgery, and Compromised Blood Supply) represents the main factors contributing to non-union in fracture neck of femur. A thorough evaluation and management plan addressing these factors are crucial for successful fracture healing.