**Core Concept**
Thyrotoxicosis is a condition characterized by an excess of thyroid hormones in the body, often resulting from Graves' disease, a common cause of hyperthyroidism. Proptosis, or exophthalmos, is a classic symptom of Graves' disease due to the infiltration of immune cells into the orbital tissues.

**Why the Correct Answer is Right**
In Graves' disease, autoantibodies stimulate the TSH receptor, leading to an overproduction of thyroid hormones. The resulting hyperthyroidism causes a variety of symptoms, including weight loss, palpitations, and exophthalmos. The autoantibodies also stimulate the production of cytokines, which cause the orbital tissues to become inflamed and swollen, leading to the protrusion of the eyeballs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it would be more indicative of hypothyroidism, which is characterized by a deficiency of thyroid hormones, not an excess.

**Option B:** This option is incorrect because it would be more indicative of a thyroid storm, a life-threatening complication of untreated or inadequately treated hyperthyroidism, but it does not explain the exophthalmos.

**Option C:** This option is incorrect because it would be more indicative of a thyroid nodule or goiter, which may cause a variety of symptoms including difficulty swallowing or breathing, but it does not explain the thyrotoxicosis or exophthalmos.

**Clinical Pearl / High-Yield Fact**
In Graves' disease, the presence of exophthalmos is a key clinical finding that distinguishes it from other causes of hyperthyroidism. The patient's history and physical examination should also include questions about weight loss, palpitations, and other symptoms of hyperthyroidism.

**Correct Answer: C. Thyroid-stimulating immunoglobulin (TSI) or thyroid-binding inhibitory immunoglobulin (TBII).**