**Core Concept**
The patient has a palpable thyroid nodule with thyrotoxic symptoms, indicating a possible case of toxic thyroid adenoma or toxic multinodular goiter. The presence of increased T4 and decreased TSH levels suggests hyperthyroidism.

**Why the Correct Answer is Right**
To confirm the diagnosis and assess the functional status of the thyroid nodule, a radioactive iodine uptake (RAIU) scan is the next best investigation. This scan measures the uptake of radioactive iodine by the thyroid gland, which can help differentiate between a functioning nodule (toxic adenoma or multinodular goiter) and a non-functioning nodule. The RAIU scan is particularly useful in identifying the hot nodule that is producing excess thyroid hormones, leading to hyperthyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** A fine-needle aspiration cytology (FNAC) is not the next best investigation in this case, as it is mainly used to assess the morphology of the thyroid nodule and diagnose malignancy, not to assess its functional status.

**Option B:** A thyroid-stimulating hormone (TSH) receptor antibody assay is useful in diagnosing Graves' disease, but it is not the next best investigation in this case, as the patient has a palpable nodule and thyrotoxic symptoms suggestive of a toxic nodule.

**Option C:** A computed tomography (CT) scan of the neck is not typically used as the next investigation in this case, as it is mainly used to assess the size and extent of the thyroid gland or to evaluate for potential complications such as thyroiditis or hemorrhage.

**Clinical Pearl / High-Yield Fact**
A hot nodule on RAIU scan is typically defined as a nodule that takes up more than 50% of the total uptake of the thyroid gland, indicating that it is producing excess thyroid hormones.

**Correct Answer: C. Computed Tomography (CT) scan of the neck is not the next best investigation in this case, as it is mainly used to assess the size and extent of the thyroid gland or to evaluate for potential complications such as thyroiditis or hemorrhage.