**Core Concept**
The patient's presentation suggests a systemic condition involving multiple organ systems, including the musculoskeletal, gastrointestinal, and renal systems. The pentad of symptoms described is classic for a condition known as **Hyperparathyroidism**.

**Why the Correct Answer is Right**
The correct investigation is likely a **Bone Scan**. Hyperparathyroidism leads to an increase in parathyroid hormone (PTH), which causes an increase in calcium levels in the blood. Elevated calcium levels stimulate osteoclasts, leading to bone resorption and pain. A bone scan would reveal areas of increased bone turnover, consistent with the patient's symptoms. The scan would show **hot spots** in areas of high bone activity, such as the bones, kidneys, and abdominal region.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a **CT Scan** would be more useful for evaluating specific lesions or abnormalities in the abdominal region, but it would not provide information on bone turnover or the systemic effects of hyperparathyroidism.
* **Option B:** A **PET Scan** is not typically used to evaluate bone turnover or the effects of hyperparathyroidism.
* **Option C:** An **MRI** would be more useful for evaluating soft tissue abnormalities, such as tumors or inflammatory conditions, but it would not provide information on bone turnover or the systemic effects of hyperparathyroidism.

**Clinical Pearl / High-Yield Fact**
Remember that hyperparathyroidism can cause a range of systemic symptoms, including kidney stones, bone pain, abdominal pain, and psychiatric symptoms. A bone scan can be an important diagnostic tool in evaluating patients with suspected hyperparathyroidism.

**Correct Answer: C. MRI.