**Core Concept**
The initial investigation of choice for a patient presenting with a solitary nodule of the thyroid gland involves assessing the functional status of the nodule and ruling out malignancy. This is crucial because thyroid nodules can be benign or malignant, and some functional nodules can cause hyperthyroidism.

**Why the Correct Answer is Right**
The correct answer is **Ultrasound of the thyroid gland**. This is because ultrasound is a non-invasive, readily available, and cost-effective imaging modality that provides detailed information about the size, shape, and composition of the thyroid nodule. It can help identify features suggestive of malignancy, such as hypoechogenicity, irregular margins, and increased vascularity. Additionally, ultrasound can guide fine-needle aspiration cytology (FNAC) if further evaluation is needed.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thyroid function tests (TFTs) are essential in the evaluation of thyroid nodules but are not the initial investigation of choice. TFTs can help identify functional nodules that cause hyperthyroidism but do not provide information about the nodule's composition or malignancy risk.
* **Option B:** CT scan of the neck is not typically used as the initial investigation for solitary thyroid nodules. While it can provide information about the nodule's relationship to adjacent structures, it exposes the patient to radiation and is more expensive than ultrasound.
* **Option C:** MRI of the neck is not commonly used for evaluating thyroid nodules. It is more expensive than ultrasound and CT and is generally reserved for specific indications, such as evaluating suspected thyroid cancer in a pregnant woman.

**Clinical Pearl / High-Yield Fact**
When evaluating a solitary thyroid nodule, it's essential to remember that the presence of a nodule does not necessarily indicate malignancy. However, nodules that are larger than 4 cm, have irregular margins, or exhibit increased vascularity on ultrasound are more likely to be malignant.

**Correct Answer: C. CT scan of the neck is not typically used as the initial investigation for solitary thyroid nodules. While it can provide information about the nodule's relationship to adjacent structures, it exposes the patient to radiation and is more expensive than ultrasound.**