**Core Concept**
The patient's presentation of unilateral proptosis that worsens with bending forward, in the absence of a thrill or bruit, suggests a condition that affects the orbit and its venous drainage. The MRI findings of a retroorbital mass with enhancement indicate a tumor or cystic lesion.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with a venous congestion syndrome, particularly in the context of a superonasal mass. The worsening of proptosis with bending forward is due to increased venous pressure in the orbit, which is exacerbated by the Valsalva maneuver. The absence of a thrill or bruit suggests that the mass is not highly vascularized, making options like carotid-cavernous fistula less likely. The MRI findings of a retroorbital mass with enhancement are consistent with a lymphoma, specifically a non-Hodgkin lymphoma, which is a common cause of orbital involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** Although thyroid ophthalmopathy can cause proptosis, it typically presents with bilateral involvement and is often associated with other signs like eyelid retraction and conjunctival injection.

**Option B:** A carotid-cavernous fistula would typically present with a bruit or thrill, which is not mentioned in the patient's history.

**Option C:** Orbital cellulitis would typically present with signs of infection, such as fever, pain, and redness, which are not mentioned in the patient's history.

**Clinical Pearl / High-Yield Fact**
In patients with unilateral proptosis, always consider the possibility of a venous congestion syndrome, particularly in the context of a superonasal mass. Remember to ask about symptoms that worsen with bending forward, as this can help distinguish between different causes of proptosis.

**Correct Answer:** C.