Final answer:
In the AP recumbent position of a sthenic patient during an upper GI examination, the gastric fundus will be filled with barium due to its superior location in the stomach and gravity's influence when the patient is lying on their back.
Step-by-step explanation:
The structure that will be filled with barium in the AP recumbent position of a sthenic patient during an upper GI examination is the gastric fundus. When a patient lies in the AP (anterior-posterior) recumbent position, gravity allows the barium to fill the superior part of the stomach, which is the fundus. The fundus of the stomach acts as a temporary storage area for undigested food and gases released during chemical digestion. Additionally, gastric emptying is regulated by the stomach and duodenum, with the fundus playing a crucial role in the process.
During an upper GI examination, it is important for the correct anatomical structures to be highlighted with barium to evaluate the patient properly. Since the fundus is located above and to the left of the esophageal connection to the stomach (cardiac region), and the sthenic body type refers to an average body habitus, the barium naturally pools in the fundus when the patient is lying on their back, making it the correct answer to the question.