**Core Concept**
Toxic multinodular goiter (TMNG) and solitary toxic nodule (STN) are common causes of hyperthyroidism in a younger population. The presence of a palpable nodule and thyrotoxic symptoms warrants further investigation to differentiate between these conditions.

**Why the Correct Answer is Right**
The next investigation would be **Fine Needle Aspiration Cytology (FNAC)** to determine the nature of the nodule, whether it is benign or malignant. FNAC is a quick, minimally invasive procedure that can provide a definitive diagnosis in most cases. It involves inserting a thin needle into the nodule to collect a sample of cells, which are then examined under a microscope for any abnormalities.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Thyroid function tests (TFTs)** are already done, and the result is known. Repeating TFTs would not provide any new information.
* **Option B:** **Scintigraphy** is a useful investigation for differentiating between TMNG and STN, but it is not the next step in this case, as the patient has already been identified as having a palpable nodule and thyrotoxic symptoms.
* **Option C:** **Thyroid ultrasound** is a useful tool for assessing thyroid nodules, but it is not as specific as FNAC in determining the nature of the nodule.

**Clinical Pearl / High-Yield Fact**
Always consider the possibility of malignancy in a palpable thyroid nodule, especially in a younger patient.

**Correct Answer: C. Thyroid ultrasound.