Which position best demonstrates a hiatal hernia during an UGI series?

The Trendelenburg position is most effective for demonstrating a hiatal hernia during an upper gastrointestinal (UGI) series because it allows gravity to assist in the visibility of the hernia by positioning the thorax higher than the abdomen. In this position, the abdominal contents are shifted downwards, which can help to make any protrusion of the stomach into the thoracic cavity more pronounced and easier to visualize on the radiographic image.

This is particularly important because hiatal hernias occur when part of the stomach pushes up through the diaphragm into the chest cavity. By using the Trendelenburg position, radiologists can enhance the chances of identifying this condition during an examination, as it helps to reveal previously hidden anatomical details.

Other positions, such as sitting upright or supine, may not provide the same clarity needed for identifying a hiatal hernia. The left lateral position, while useful for other evaluations, does not utilize gravity in a way that would enhance the visibility of a hiatal hernia.