**Core Concept**
Grave's ophthalmopathy is an autoimmune condition associated with Graves' disease, characterized by inflammation of orbital tissues, leading to swelling of the extraocular muscles and orbital fat. This results in progressive bulging of the eyes, a hallmark clinical feature.

**Why the Correct Answer is Right**
Proptosis (bulging of the eyes) is the most common and defining feature of Grave's ophthalmopathy. It occurs due to infiltration of lymphocytes and fibroblasts into the orbital tissues, causing edema and thickening of orbital fat and muscles. This increases the anterior orbital volume, leading to visible proptosis, especially in the morning. It is often accompanied by diplopia, pain, and eyelid retraction.

**Why Each Wrong Option is Incorrect**
Option B: Ptosis is not a primary feature of Grave's ophthalmopathy. It is more commonly associated with cranial nerve III palsy or myasthenia gravis.
Option C: Reduced intraocular tension is not a feature; in fact, increased orbital pressure from swelling can impair intraocular pressure regulation.
Option D: Increased power of convergence is not a recognized clinical sign in this condition; convergence disorders are unrelated to orbital inflammation.

**Clinical Pearl / High-Yield Fact**
"Proptosis is the cardinal sign of Grave’s ophthalmopathy—remember it with the mnemonic: **'Grave's = Googly eyes'** (bulging eyes)."

✓ Correct Answer: A. Proptosis