**Core Concept**
Delayed union of a fracture following surgical treatment is a complication that can arise due to inadequate blood supply to the bone fragments, impaired bone healing mechanisms, or other factors affecting the fracture site. This condition is often associated with the failure of fracture stabilization and healing, leading to prolonged recovery times.

**Why the Correct Answer is Right**
The most common cause of delayed union in a fracture following surgical treatment is **insufficient blood supply** to the bone fragments. This can occur due to **inadequate revascularization** of the bone tissue after trauma or surgery, leading to impaired bone healing mechanisms. Additionally, factors such as **infection**, **malunion**, or **non-compliance** with post-operative care can also contribute to delayed union.

**Why Each Wrong Option is Incorrect**
* **Option A:** While **osteoporosis** can contribute to delayed union, it is not the primary cause in the context of surgical treatment.
* **Option B:** **Malunion** is a complication of fracture healing, but it is not directly related to delayed union.
* **Option C:** **Infection** can contribute to delayed union, but it is not the most common cause.

**Clinical Pearl / High-Yield Fact**
When managing patients with fractures, it is essential to maintain a high index of suspicion for delayed union, particularly in cases where surgical treatment has been performed. This can be achieved by closely monitoring fracture healing progress, ensuring adequate blood supply to the bone fragments, and addressing any underlying factors that may be contributing to delayed union.

**Correct Answer:** C. Infection.