## Core Concept
The investigation of choice for whole-body imaging in metastasis involves identifying the most effective imaging modality that can accurately detect and monitor metastatic lesions across the entire body. This requires an imaging technique with high sensitivity, specificity, and the ability to image multiple regions simultaneously.

## Why the Correct Answer is Right
**PET-CT (Positron Emission Tomography-Computed Tomography)** is widely regarded as the investigation of choice for whole-body imaging in metastasis. This is because PET-CT combines the functional information of PET, which highlights areas of high metabolic activity typical of malignant tissues, with the anatomical detail provided by CT. This combination allows for the detection of metastatic lesions that might be missed by other imaging modalities, and it helps in staging, monitoring treatment response, and detecting recurrence. The glucose analog **FDG (Fluorodeoxyglucose)** is commonly used in PET-CT, as many cancer cells exhibit increased glucose uptake and metabolism.

## Why Each Wrong Option is Incorrect
- **Option A: Ultrasound** is not suitable for whole-body imaging due to its limited depth penetration, operator dependency, and inability to image certain areas like the brain or lungs effectively.
- **Option B: MRI (Magnetic Resonance Imaging)**, while excellent for soft tissue characterization and certain types of lesions, is not typically used for whole-body screening due to longer examination times, higher costs, and lesser availability compared to CT or PET-CT.
- **Option D: Bone Scan** is specifically used for detecting bone metastases and is not suitable for whole-body imaging of all types of metastases.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **PET-CT with FDG** is particularly useful in oncology for its ability to provide both anatomical and functional information, making it superior for detecting metastases in many cancer types. However, it's essential to note that its utility can vary depending on the type of cancer, with some cancers being less FDG-avid.

## Correct Answer: . PET-CT.