**Core Concept**
Fat-embolism syndrome (FES) is a rare but life-threatening condition caused by the release of fat globules into the bloodstream, leading to pulmonary and systemic complications. This condition is often associated with long bone fractures, particularly those of the femur.

**Why the Correct Answer is Right**
The commonest site of fracture leading to fat-embolism is the **femur**. This is because the femur has a large amount of fatty marrow, which can be released into the bloodstream after a fracture. The femur's long bone structure and rich blood supply make it more susceptible to fat embolism syndrome. The mechanism of FES involves the release of fat globules into the venous system, which can then travel to the lungs, brain, and other organs, causing damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** The humerus is a long bone, but it is not as commonly associated with fat-embolism syndrome as the femur.
* **Option B:** The pelvis is a complex bone structure, but fractures here are not typically associated with FES.
* **Option C:** The tibia is another long bone, but it is not the most common site for fat-embolism syndrome.

**Clinical Pearl / High-Yield Fact**
When managing patients with long bone fractures, especially those of the femur, it is essential to be aware of the risk of fat-embolism syndrome and monitor for symptoms such as respiratory distress, neurological changes, and hypoxia.

**Correct Answer:** C.