Final answer:
The common bile duct is the duct most commonly obstructed by gallstones. This blockage can cause jaundice and pain due to the accumulation of bile and bilirubin.
Step-by-step explanation:
The duct that is commonly obstructed when gallstones are present is the common bile duct. Gallstones are formed when a large amount of cholesterol accumulates in the gallbladder, resulting in solid particles that can obstruct the flow of bile. This obstruction can cause pain, and if the common bile duct is blocked, bile and bilirubin cannot be excreted properly, leading to jaundice — a yellowing of the skin and the whites of the eyes.
The common bile duct is a vital structure formed by the union of the common hepatic duct and the gallbladder's cystic duct, which allows bile to flow from the liver, where it is produced, to the small intestine, where it plays a critical role in digestion, particularly in the emulsification of fats.