**Question:** Most common cause of unilateral proptosis is-

A. Graves' disease
B. Hageman factor deficiency
C. Carcinoma of the orbit
D. Primary angle closure glaucoma

**Core Concept:**
Proptosis refers to the protrusion of the eyeball beyond the normal orbital boundaries, caused by an enlarged orbital mass or orbital bone abnormalities. The most common cause of unilateral proptosis depends on the underlying pathology.

**Why the Correct Answer is Right:**
Graves' disease (hyperthyroidism) is a common autoimmune disorder that involves thyroid hormones and thyroid-stimulating immunoglobulins (TSIs). In the orbit, the hyperthyroid state leads to increased lymphatic and vascular permeability, causing inflammation and edema, which results in proptosis.

**Why Each Wrong Option is Incorrect:**

B. Hageman factor deficiency (coagulation disorder) is unrelated to orbital inflammation and proptosis.
C. Carcinoma of the orbit is a malignant tumor that can cause proptosis, but it is less common than Graves' disease.
D. Primary angle closure glaucoma is a glaucoma condition that primarily affects the anterior segment of the eye, not the orbit. It doesn't cause proptosis.

**Clinical Pearl:**
Graves' disease is a crucial differential diagnosis for unilateral proptosis, especially in the context of thyroid involvement. Recognizing the association between proptosis and thyroid disorders is essential for accurate diagnosis and timely management.

**Correct Answer:** A. Graves' disease (hyperthyroidism) is the most common cause of unilateral proptosis due to orbital inflammation and edema secondary to thyroid hormones and thyroid-stimulating immunoglobulins (TSIs).