Investigation of choice for choledocolithiasis is

**Core Concept**
Choledocolithiasis refers to the presence of gallstones in the common bile duct. Accurate diagnosis and investigation of choledocolithiasis are crucial to prevent complications such as jaundice, pancreatitis, and cholangitis. The investigation of choice should provide a clear image of the biliary tree and detect any stones or strictures.

**Why the Correct Answer is Right**
The investigation of choice for choledocolithiasis is **Endoscopic Retrograde Cholangiopancreatography (ERCP)**. ERCP is a powerful tool that combines endoscopy and fluoroscopy to visualize the biliary and pancreatic ducts. It allows for the direct visualization of the common bile duct and the detection of any stones or strictures. ERCP also enables therapeutic interventions such as balloon dilation and stenting of strictures, and removal of stones. The high-resolution images provided by ERCP make it an ideal investigation for diagnosing and managing choledocolithiasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is a useful initial investigation for gallbladder disease, but it may not always detect small stones or those located in the common bile duct. **Option B:** Magnetic Resonance Cholangiopancreatography (MRCP) is a non-invasive imaging technique that can detect stones and strictures in the biliary tree, but it may not provide the same level of detail as ERCP. **Option D:** Computed Tomography (CT) scan can detect stones and strictures in the biliary tree, but it is not as sensitive as ERCP for detecting small stones or those located in the common bile duct.

**Clinical Pearl / High-Yield Fact**
ERCP is a powerful tool for diagnosing and managing choledocolithiasis, but it carries a higher risk of complications such as pancreatitis and bleeding compared to other imaging modalities. It is essential to carefully weigh the benefits and risks of ERCP before proceeding with the investigation.

**Correct Answer: C. ERCP**