**Core Concept**
Thyrotoxicosis in childhood, also known as hyperthyroidism, is a condition characterized by an excess of thyroid hormones. The most common cause of thyrotoxicosis in children is a condition that leads to the overproduction of thyroid hormones. This condition often results from an autoimmune response or a genetic mutation affecting the thyroid gland.

**Why the Correct Answer is Right**
The most common cause of thyrotoxicosis in childhood is **Graves' disease**. This autoimmune disorder leads to the production of autoantibodies that stimulate the thyroid-stimulating hormone receptor (TSHR), causing the thyroid gland to produce excessive amounts of thyroid hormones, including triiodothyronine (T3) and thyroxine (T4). The increased thyroid hormone production leads to symptoms such as weight loss, palpitations, and exophthalmos.

**Why Each Wrong Option is Incorrect**
**Option A:** Subacute thyroiditis is an inflammatory condition that can cause thyrotoxicosis, but it is less common in childhood than Graves' disease.
**Option B:** Toxic multinodular goiter is a condition characterized by multiple nodules in the thyroid gland, but it is rare in children and typically occurs in older adults.
**Option C:** Thyroiditis, including subacute and chronic forms, can cause thyrotoxicosis, but it is not the most common cause in childhood.

**Clinical Pearl / High-Yield Fact**
Graves' disease often presents with exophthalmos (bulging eyes), which is a classic sign of the condition. This symptom is often remembered using the mnemonic "Graves' disease = GIGANTIC eyes."

**Correct Answer:** A. Graves' disease.