**Core Concept**
The clinical presentation of flank pain radiating to the groin region is suggestive of ureteral colic, likely due to a kidney stone (nephrolithiasis) causing ureteral obstruction. This condition requires prompt investigation to identify the cause and prevent potential complications.

**Why the Correct Answer is Right**
The first investigation to be done in this case is an **abdominal X-ray** (Option C). This imaging modality can help identify the presence of a kidney stone, which is the most common cause of ureteral colic. An abdominal X-ray can also help rule out other causes of flank pain, such as bowel obstruction or pneumothorax. Additionally, an abdominal X-ray can provide useful information about the size and location of the kidney stone, which can guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** An ultrasound is a useful imaging modality in the evaluation of kidney stones, but it is not the first line of investigation. Ultrasound can be used to identify kidney stones, but it may not be as sensitive as an abdominal X-ray in detecting smaller stones.
**Option B:** A CT scan is a more sensitive imaging modality than an abdominal X-ray, but it is not typically used as the first investigation due to its higher cost and radiation exposure.
**Option D:** A urinalysis is a useful tool in the evaluation of kidney stones, but it is not an imaging modality and therefore not the first investigation to be done.

**Clinical Pearl / High-Yield Fact**
The classic presentation of ureteral colic due to a kidney stone is the "three Ps": pain, paresthesias (numbness or tingling), and polyuria (increased urine production). This clinical correlation can help guide the diagnosis and management of patients with suspected kidney stones.

**Correct Answer: C. Abdominal X-ray.**