## **Core Concept**
The investigation of choice for gallstones involves imaging techniques that can accurately visualize the gallbladder and detect stones. The most commonly used modalities include ultrasound, CT scan, and MRI. Among these, **ultrasound** is widely recognized for its high sensitivity and specificity in detecting gallstones.

## **Why the Correct Answer is Right**
Ultrasound is considered the investigation of choice for gallstones because it is **non-invasive**, **widely available**, and **cost-effective**. It uses high-frequency sound waves to produce images of the gallbladder and can detect stones as small as 1-2 mm in diameter. The sensitivity and specificity of ultrasound for detecting gallstones are high, making it an excellent initial imaging test.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a CT scan can detect gallstones, it is not the first-line imaging modality due to higher costs, radiation exposure, and lower sensitivity for small stones compared to ultrasound.
- **Option B:** Although an MRI can provide detailed images of the gallbladder and bile ducts, it is more expensive and not routinely needed for the initial diagnosis of gallstones.
- **Option D:** This option is not specified, but generally, other imaging techniques like X-rays or plain abdominal radiographs are not used for diagnosing gallstones as they are less sensitive and specific.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that ultrasound is not only useful for detecting gallstones but also for identifying signs of complications such as cholecystitis. The presence of **Murphy's sign** (pain upon palpation of the gallbladder during inspiration) and **sonographic Murphy's sign** (cessation of breathing during ultrasound probe pressure) are clinical and ultrasound findings that suggest acute cholecystitis.

## **Correct Answer:** . Ultrasound