**Core Concept**
Toxic multinodular goiter (TMNG) and toxic solitary nodule (TSN) are both causes of thyrotoxicosis due to autonomous thyroid hormone production, but they have distinct clinical and pathological features. The key here is to differentiate between these two conditions based on the RAIU scan results.

**Why the Correct Answer is Right**
A RAIU scan measures the uptake of radioactive iodine by the thyroid gland, reflecting thyroid function and the presence of functioning thyroid tissue. In hyperthyroidism due to a toxic solitary nodule (TSN), the affected nodule takes up more radioactive iodine than the surrounding normal thyroid tissue, resulting in an increased RAIU scan. This is because the TSN has autonomous, thyroid-stimulating hormone (TSH)-independent function.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxic multinodular goiter (TMNG) would typically show a diffusely increased RAIU scan, not a focal increase as seen in TSN.

**Option B:** Graves' disease is another cause of hyperthyroidism, but it is characterized by diffuse thyroid gland enlargement and a diffusely increased RAIU scan, not a focal increase.

**Option D:** Subacute thyroiditis is an inflammatory condition of the thyroid gland that can cause thyrotoxicosis, but it is not associated with an increased RAIU scan.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that a focal increase in RAIU scan typically indicates a toxic solitary nodule (TSN), whereas a diffusely increased RAIU scan suggests Graves' disease or toxic multinodular goiter.

**Correct Answer:** C.