**Core Concept**
The patient's presentation is suggestive of a retro-orbital process causing increased pressure and swelling behind the globe, leading to proptosis, pain, and limited extraocular movement. This could be due to an infection, tumor, or hematoma.

**Why the Correct Answer is Right**
The investigation of choice in this scenario is a CT scan with contrast to evaluate for an orbital abscess or hematoma. This imaging modality allows for the assessment of the extent of the process, involvement of surrounding structures, and potential complications such as cavernous sinus thrombosis. The contrast helps to differentiate between soft tissue and fluid collections, making it essential for accurate diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is not the best choice for evaluating retro-orbital processes, as it may not provide sufficient detail for assessing surrounding structures and potential complications.

**Option B:** MRI would be a better choice for soft tissue evaluation, but it may not be readily available in all settings and is generally more expensive than CT.

**Option C:** Orbital CT without contrast would not provide sufficient information to differentiate between soft tissue and fluid collections, making it less useful for diagnosing orbital abscess or hematoma.

**Clinical Pearl / High-Yield Fact**
In cases of suspected orbital infection or hematoma, it's essential to consider the patient's immunocompromised status, recent trauma or surgery, and potential sources of infection, such as sinusitis or dental abscess.

**Correct Answer:** C.