**Core Concept**
Fat embolism syndrome (FES) is a serious complication that can occur in trauma patients, particularly those with long bone fractures. It is characterized by the presence of fat globules in the bloodstream, which can lead to respiratory, neurological, and cardiovascular complications.

**Why the Correct Answer is Right**
Mobility of joint is a significant risk factor for fat embolism syndrome. When a long bone is fractured, the marrow cavity is disrupted, releasing fatty acids into the bloodstream. The mobility of the joint allows for the spread of these fatty acids to the lungs, brain, and other organs, increasing the risk of FES. This is because the movement of the joint can lead to the release of fat globules into the bloodstream, which can then travel to the lungs and other organs, causing damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Diabetes Mellitus is not a direct risk factor for fat embolism syndrome. While diabetes can increase the risk of infection and other complications, it is not directly related to the pathophysiology of FES.

**Option B:** Hypovolemic shock is a potential complication of trauma, but it is not a direct risk factor for fat embolism syndrome. In fact, hypovolemic shock can actually decrease the risk of FES by reducing the blood volume and reducing the pressure in the bloodstream.

**Option C:** Respiratory failure is a potential complication of fat embolism syndrome, but it is not a risk factor for FES. Respiratory failure can occur as a result of FES, but it is not a factor that contributes to the development of the syndrome.

**Option D:** Hypovolemic shock is the same as Option B, which is incorrect.

**Clinical Pearl / High-Yield Fact**
When assessing a trauma patient with a long bone fracture, it is essential to consider the risk of fat embolism syndrome, particularly if the patient is mobile or has a high-energy injury.

**Correct Answer:** B. Mobility of joint