**Core Concept**
Elevated conjugated bilirubin in a patient with choledocholithiasis suggests an obstruction at or below the level of the common bile duct, which could be caused by an ampullary obstructive calculus. The ampulla of Vater is the location where the bile and pancreatic ducts empty into the small intestine.

**Why the Correct Answer is Right**
Ampullary obstructive calculi can be difficult to diagnose on imaging alone. **Endoscopic retrograde cholangiopancreatography (ERCP)** is a sensitive investigation for detecting ampullary obstruction, as it allows for direct visualization of the ampulla and the biliary and pancreatic ducts. ERCP can also facilitate the removal of the calculus. The procedure involves inserting a flexible tube through the mouth and into the small intestine, followed by injection of contrast material to visualize the biliary and pancreatic ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** Magnetic Resonance Cholangiopancreatography (MRCP) is a non-invasive imaging technique that can show the biliary and pancreatic ducts. However, it may not be as sensitive as ERCP in detecting small ampullary calculi.

**Option B:** Computed Tomography (CT) scan of the abdomen may show a dilated biliary system but is not as sensitive as ERCP in detecting ampullary obstruction.

**Option C:** Ultrasound is often used as a first-line imaging modality for gallbladder disease but may not be as sensitive as ERCP in detecting ampullary obstruction.

**Option D:** Magnetic Resonance Imaging (MRI) of the abdomen may show a dilated biliary system but is not as sensitive as ERCP in detecting ampullary obstruction.

**Clinical Pearl / High-Yield Fact**
Ampullary obstruction can cause severe abdominal pain, jaundice, and pancreatitis. Early diagnosis and removal of the calculus are essential to prevent complications.

**Correct Answer:** C. ERCP is the most sensitive investigation for detecting ampullary obstructive calculi.