The common bile duct (CBD) is the most frequent site of obstruction by gallstones because it is the channel through which bile drains from the liver and gallbladder into the duodenum. Gallstones formed in the gallbladder can migrate and lodge in the CBD, causing choledocholithiasis, which leads to biliary colic, jaundice, and cholangitis. The cystic duct and gallbladder neck are commonly involved in gallstone disease, but obstruction in these areas typically causes different clinical presentations, such as acute cholecystitis. Intrahepatic duct stones are much less common. Surgical management often requires CBD exploration or endoscopic removal to relieve the obstruction.

Reference: Sabiston Textbook of Surgery, 21st Edition.