**Core Concept**
Diffuse toxic goiter, also known as Graves' disease, is an autoimmune disorder that leads to hyperthyroidism. In this condition, the thyroid gland becomes enlarged and overproduces thyroid hormones due to the stimulation of the thyroid-stimulating hormone receptor by thyroid-stimulating immunoglobulins.

**Why the Correct Answer is Right**
Graves' disease is characterized by the presence of thyroid-stimulating antibodies that stimulate the thyroid-stimulating hormone receptor, leading to increased production of thyroid hormones. This results in hyperthyroidism, which is associated with symptoms such as weight loss, tremors, and palpitations. The thyroid gland becomes diffusely enlarged due to the increased production of thyroid hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the characteristic features of diffuse toxic goiter.
**Option B:** This option is incorrect because it is more commonly associated with hypothyroidism.
**Option C:** This option is incorrect because it is a type of thyroiditis that is typically associated with pain and tenderness in the thyroid gland.

**Clinical Pearl / High-Yield Fact**
A key feature of Graves' disease is the presence of thyroid-stimulating antibodies, which can be detected using a radioimmunoassay. This is a useful diagnostic tool that can help differentiate Graves' disease from other causes of hyperthyroidism.

**Correct Answer:** C.