## Core Concept
The question pertains to the selection of the most appropriate imaging modality, specifically ultrasound, for various clinical conditions. Ultrasound is a versatile, non-invasive imaging technique widely used for its safety, cost-effectiveness, and real-time imaging capabilities.

## Why the Correct Answer is Right
Ultrasound is particularly valuable in **pancreatitis** and **gallstones** due to its high sensitivity and specificity for detecting gallstones and assessing pancreatic morphology. It is also the first-line imaging modality for evaluating gallbladder disease. Ultrasound can help identify gallstones, gallbladder inflammation, and pancreatitis by directly visualizing the pancreas and gallbladder.

## Why Each Wrong Option is Incorrect
- **Option A:** While ultrasound can be used for liver conditions, it's not exclusively the investigation of choice for all liver diseases; other modalities like CT, MRI, or liver-specific contrast-enhanced ultrasound may be preferred depending on the condition.
- **Option B:** Ultrasound is not typically the first-line imaging modality for diagnosing **pulmonary embolism**; CT pulmonary angiography (CTPA) is the gold standard.
- **Option D:** For **appendicitis**, while ultrasound can be helpful, especially in pediatric patients or to reduce radiation exposure, CT scan with contrast is often considered more accurate, especially in adults.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that ultrasound is the **investigation of choice for suspected gallstones and acute pancreatitis** due to its accuracy, safety, and ability to provide real-time imaging. Remember, the mnemonic "RUQ ultrasound" can help recall that Right Upper Quadrant (RUQ) pain often prompts an ultrasound to evaluate for gallstones and biliary obstruction.

## Correct Answer: C.