## **Core Concept**
Intermittent proptosis refers to the temporary protrusion of the eyeball, which can be associated with various conditions. This symptom is often linked to vascular or orbital abnormalities. Understanding the causes of intermittent proptosis is crucial for diagnosing and managing underlying conditions.

## **Why the Correct Answer is Right**
The correct answer, **Carotico-cavernous fistula**, is a condition characterized by an abnormal connection between the carotid arterial system and the cavernous sinus. This fistula can cause intermittent proptosis due to increased pressure and blood volume within the orbital veins, leading to swelling and protrusion of the eyeball. The intermittent nature of the proptosis can be attributed to fluctuations in blood flow through the fistula.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While orbital tumors can cause proptosis, they typically result in a more gradual and progressive protrusion of the eyeball, rather than intermittent.
- **Option B:** Thyroid ophthalmopathy can cause proptosis, but it is usually bilateral and associated with other symptoms such as eyelid retraction and restrictive myopathy, rather than being intermittent.
- **Option C:** Orbital varix is a vascular malformation that can indeed cause intermittent proptosis due to changes in venous pressure or during Valsalva maneuver, but it is not as directly associated with the symptom as a carotico-cavernous fistula.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that intermittent proptosis can be exacerbated or triggered by a Valsalva maneuver, which increases venous pressure and can worsen symptoms in conditions like orbital varix or carotico-cavernous fistula. Recognizing this association can aid in diagnosis.

## **Correct Answer:** D. Carotico-cavernous fistula