**Core Concept:** Suspected gallstones are a common clinical scenario, which often presents with upper abdominal pain, jaundice, or cholangitis. Diagnosis relies on imaging techniques to visualize gallstones in the biliary tract.

**Why the Correct Answer is Right:** Ultrasonography (USG) is the initial investigation of choice for evaluating a suspected case of gallstones. It is a non-invasive, radiation-free, widely available, and cost-effective technique. It can visualize the gallbladder, cystic duct, and common bile duct (CBD) to detect gallstones and associated complications (e.g., cholecystitis, cholecystoduodenal fistula).

**Why Each Wrong Option is Incorrect:**

A. Abdominal X-ray: This imaging modality is limited in detecting gallstones due to its poor sensitivity (50-60%) and insensitivity to detecting biliary tree abnormalities.

B. Computed tomography (CT): Although CT has higher sensitivity (90-95%) for detecting gallstones, it is not the initial choice because of radiation exposure, higher cost, and limited availability.

C. Endoscopic retrograde cholangiopancreatography (ERCP): For gallstone evaluation, ERCP is invasive, time-consuming, and requires specialized equipment and expertise.

D. Blood tests: Blood tests (e.g., liver function tests) primarily assess liver function, inflammation, or cholestasis secondary to gallstones, not directly detecting gallstones themselves.

**Clinical Pearl:** Ultrasonography (USG) is the initial choice for evaluating gallstones as it is non-invasive, radiation-free, widely available, and cost-effective. Its sensitivity ranges from 90-95%, making it highly accurate for detecting gallstones and associated complications.