**Core Concept**
The question is testing the knowledge of the common causes of unilateral proptosis, which is the protrusion of one eye from the orbit. Thyrotoxicosis is a condition characterized by an overproduction of thyroid hormones, leading to various systemic and ocular manifestations.

**Why the Correct Answer is Right**
Thyrotoxicosis is the most common cause of unilateral proptosis due to the inflammatory and vascular changes it induces. The increased levels of thyroid hormones lead to increased permeability of the orbital vessels, resulting in exudation of fluid into the orbital tissues. This causes swelling and protrusion of the eye, a condition known as exophthalmos. The inflammatory response also leads to edema and fibrosis of the orbital tissues, further contributing to the proptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoblastoma is a malignant tumor of the retina and is not typically associated with proptosis. While it can cause leukocoria (white pupillary reflex) and strabismus, it does not usually cause unilateral proptosis.

**Option B:** Intraocular hemorrhage can cause sudden vision loss and pain, but it is not typically associated with proptosis. The hemorrhage is usually confined to the eye and does not cause significant orbital tissue swelling.

**Option D:** Raised intracranial tension can cause papilledema (swelling of the optic disc) and optic atrophy, but it is not typically associated with unilateral proptosis. The increased intracranial pressure can cause the eye to bulge, but this is usually a bilateral finding.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that thyrotoxicosis can cause a range of ocular manifestations, including exophthalmos, lid retraction, and cataracts. A high index of suspicion for thyrotoxicosis should be maintained in patients presenting with unilateral proptosis, especially if there are other systemic symptoms such as weight loss, tremors, and palpitations.

**✓ Correct Answer: A. Thyrotoxicosis**