## Core Concept
The question tests the understanding of imaging modalities used in oncology, specifically for detecting distant metastases in pancreatic cancer. Pancreatic cancer is known for its aggressive nature and high potential for metastasis. Accurate staging is crucial for determining the prognosis and treatment plan.

## Why the Correct Answer is Right
**Option D: PET/CT (Positron Emission Tomography/Computed Tomography)** is the best imaging modality for detecting distant metastases in pancreatic cancer. PET/CT combines the functional information of PET, which highlights areas of high metabolic activity typical of malignant cells, with the anatomical details provided by CT. This combination allows for the detection of primary tumors, regional lymph nodes, and distant metastases. PET/CT is particularly useful in pancreatic cancer because it can identify metastases that might not be evident on CT or MRI alone, thus altering the treatment plan.

## Why Each Wrong Option is Incorrect
- **Option A: Ultrasound** is often used as an initial imaging modality for pancreatic cancer due to its non-invasive nature and availability. However, it has limitations in detecting distant metastases, especially in obese patients or when the tumor is located in difficult-to-image areas.
- **Option B: CT (Computed Tomography)** is commonly used for staging pancreatic cancer and can detect some metastases. However, it might miss small metastatic lesions that are not yet significantly different in size or density from surrounding tissues.
- **Option C: MRI (Magnetic Resonance Imaging)** provides excellent soft-tissue contrast and is useful for evaluating the primary tumor and local resectability. While MRI can detect liver metastases effectively, it is not as widely available or as sensitive as PET/CT for detecting distant metastases.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **PET/CT** is increasingly being used in oncology for staging and restaging due to its ability to provide both functional and anatomical information. For pancreatic cancer, PET/CT can help in identifying patients who might benefit from surgery or other interventions by accurately detecting distant metastases.

## Correct Answer: D. PET/CT.