Which parts of the duodenum are mobile?

The correct choice highlights that the 1st part and part of the 4th part of the duodenum are mobile. The duodenum is divided into four parts: the 1st part is the superior segment, which is highly mobile and can move according to the position of the stomach and other surrounding structures. This mobility allows it to adapt to changes in body posture and the position of other organs.

The 4th part also has some mobility because it can ascend to meet the jejunum and is connected to structures that allow for slight movements. In contrast, the 2nd and 3rd parts are generally more fixed in position as they are retroperitoneal, meaning they lie behind the peritoneum and are less free to move.

This anatomical arrangement plays a significant role in the overall function of the digestive system, as it allows the duodenum to effectively process the chyme from the stomach and regulate the flow of digestive enzymes and bile.