A 35 yr old male presented with a 15 day history of proptosis in his right eye and pain on eye movement. There is difficulty in upward and downward gaze movements. CT scan showed a cystic lesion with a hyperdense opacity within it, located in the superior oblique muscle. What is the MOST probable diagnosis?

**Core Concept**
The case presents a patient with proptosis, pain on eye movement, and limitation of gaze movements, indicating a possible orbital and ocular muscle involvement. The CT scan findings of a cystic lesion with a hyperdense opacity within it, located in the superior oblique muscle, suggest a specific diagnosis related to a parasitic infection.

**Why the Correct Answer is Right**
The presence of a cystic lesion with a hyperdense opacity in the superior oblique muscle is highly suggestive of a cysticercosis, a parasitic infection caused by the pork tapeworm *Taenia solium*. The cysticercus larvae can invade various muscles, including the extraocular muscles, leading to localized inflammation, scarring, and fibrosis. In this case, the involvement of the superior oblique muscle would explain the difficulty in upward and downward gaze movements.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option would be incorrect as it does not explain the specific involvement of the superior oblique muscle or the presence of a cystic lesion with a hyperdense opacity.
* **Option B:** This option is not relevant to the clinical presentation of proptosis, pain on eye movement, and limitation of gaze movements.
* **Option C:** This option would be incorrect as it does not account for the parasitic nature of the infection or the involvement of the superior oblique muscle.

**Clinical Pearl / High-Yield Fact**
Cysticercosis can involve any muscle in the body, including extraocular muscles, and can cause a range of symptoms depending on the location and size of the cysts. In this case, the involvement of the superior oblique muscle would explain the specific limitation of gaze movements.

**Correct Answer: C. Cysticercosis of the superior oblique muscle**

✓ Correct Answer: D. Cysticercosis cellulosae