**Core Concept**
The initial investigation of choice for a patient presenting with a solitary nodule of the thyroid involves assessing the nodule's functionality and potential malignancy. This typically involves evaluating the presence of thyrotropin (thyroid-stimulating hormone, TSH) and the thyroid hormone levels to determine if the nodule is functioning autonomously.

**Why the Correct Answer is Right**
Fine-needle aspiration cytology (FNAC) is the preferred initial investigation for a solitary thyroid nodule, as it provides valuable information about the nodule's cellular characteristics and potential malignancy. FNAC involves inserting a thin needle into the nodule under ultrasound guidance to collect a sample of cells for histopathological examination. This procedure helps in distinguishing between benign and malignant nodules, guiding further management. The American Thyroid Association recommends FNAC as the initial diagnostic test for solitary thyroid nodules.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is an essential tool in evaluating thyroid nodules, but it is not the initial investigation of choice. Ultrasound can provide information about the nodule's size, location, and echogenicity, but it may not provide a definitive diagnosis of malignancy.
**Option B:** Serum calcitonin levels are used to diagnose medullary thyroid carcinoma, but they are not the initial investigation of choice for a solitary nodule.
**Option C:** Serum TSH and thyroid hormone levels are essential in evaluating thyroid function, but they do not provide information about the nodule's cellular characteristics or potential malignancy.

**Clinical Pearl / High-Yield Fact**
Fine-needle aspiration cytology (FNAC) has a high sensitivity and specificity for diagnosing papillary thyroid carcinoma, the most common type of thyroid cancer.

**Correct Answer:** C. Fine-needle aspiration cytology (FNAC)