**Core Concept**
Intravenous cholecystography (IVCG) and ultrasound are commonly used for diagnosing gallstones. However, the investigation of choice for gallstones is ultrasound due to its non-invasive nature, lack of radiation, and high sensitivity for detecting gallstones.

**Why the Correct Answer is Right**
Ultrasound uses high-frequency sound waves to produce images of the gallbladder and bile ducts, allowing for the detection of gallstones, sludge, and other abnormalities. The probe is placed on the patient's skin, and the sound waves are transmitted through the body, producing images on a screen. Ultrasound is particularly useful for detecting gallstones in patients with kidney disease or those who are allergic to iodine, as it does not use contrast agents.

**Why Each Wrong Option is Incorrect**
**Option A:** Intravenous cholecystography (IVCG) is an older, less commonly used test that involves injecting a contrast agent into the bloodstream, which is then excreted by the liver and concentrated in the gallbladder. While it can detect gallstones, it is less sensitive than ultrasound and involves radiation and contrast agents.

**Option B:** Computed Tomography (CT) scan is a more expensive and less sensitive test for gallstones than ultrasound, and it involves radiation, making it a less ideal choice for initial diagnosis.

**Option C:** Magnetic Resonance Cholangiopancreatography (MRCP) is a more expensive and less commonly used test for gallstones, and it is not as sensitive as ultrasound for detecting small gallstones.

**Clinical Pearl / High-Yield Fact**
Ultrasound is the first-line imaging modality for diagnosing gallstones due to its non-invasive nature, lack of radiation, and high sensitivity.

**Correct Answer: A. Ultrasound**