**Core Concept**
Grave's thyrotoxicosis is a medical emergency caused by an overproduction of thyroid hormones, primarily triiodothyronine (T3) and thyroxine (T4), due to autoimmune stimulation of the thyroid gland. The definitive treatment involves reducing thyroid hormone levels and suppressing thyroid hormone production.

**Why the Correct Answer is Right**
Subtotal or near-total thyroidectomy is considered the definitive treatment for Grave's thyrotoxicosis in patients who are symptomatic or have failed medical therapy. This surgical approach involves removing a significant portion of the thyroid gland, thereby reducing thyroid hormone production. The procedure is typically performed by an experienced surgeon to minimize complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Radioactive iodine ablation is a common treatment for hyperthyroidism but is not considered definitive for Grave's thyrotoxicosis, as it may not be effective in all cases and can take several weeks to months to achieve euthyroidism.

**Option B:** Methimazole is a thionamide medication used to reduce thyroid hormone production by inhibiting the synthesis of T3 and T4. While it is effective in managing hyperthyroidism, it is not a definitive treatment for Grave's thyrotoxicosis and may require long-term therapy.

**Option C:** Propylthiouracil is another thionamide medication used to treat hyperthyroidism by inhibiting the synthesis of T3 and T4. Like methimazole, it is not a definitive treatment for Grave's thyrotoxicosis and may require long-term therapy.

**Clinical Pearl / High-Yield Fact**
In patients with severe thyrotoxicosis, it is essential to promptly initiate treatment to prevent cardiovascular complications, such as arrhythmias and heart failure. A high index of suspicion for Grave's thyrotoxicosis should be maintained in patients with symptoms such as weight loss, palpitations, and exophthalmos.

**Correct Answer:** C. Subtotal or near-total thyroidectomy is considered the definitive treatment for Grave's thyrotoxicosis in patients who are symptomatic or have failed medical therapy.