**Core Concept**
Endoscopic retrograde cholangiopancreatography (ERCP) and magnetic resonance cholangiopancreatography (MRCP) are imaging modalities used to diagnose common bile duct (CBD) stones. ERCP is a more invasive procedure that allows for therapeutic interventions, whereas MRCP is a non-invasive technique providing detailed images of the biliary tree.

**Why the Correct Answer is Right**
MRCP is the preferred imaging modality for diagnosing CBD stones due to its non-invasive nature and excellent visualization of the biliary anatomy. It can detect stones, strictures, and other abnormalities in the biliary tree without the need for contrast agents or radiation exposure. MRCP is particularly useful for patients with suspected CBD stones and a history of previous abdominal surgery or pancreatitis, as it can help identify complications such as bile duct strictures or dilatation.

**Why Each Wrong Option is Incorrect**
**Option A:** ERCP is not the most appropriate initial diagnostic test for CBD stones because it is an invasive procedure that carries risks of complications such as pancreatitis and bleeding.
**Option B:** Ultrasound is not sensitive enough to diagnose small CBD stones and may not provide sufficient detail of the biliary anatomy.
**Option C:** CT scan with contrast is not typically used to diagnose CBD stones as it may not provide sufficient detail of the biliary tree and may not be as sensitive as MRCP.

**Clinical Pearl / High-Yield Fact**
MRCP is a non-invasive, radiation-free imaging modality that provides excellent visualization of the biliary tree, making it an ideal first-line diagnostic test for suspected CBD stones.

**Correct Answer: D. MRCP**