Final answer:
Barium sulfate is the best oral contrast agent for opacifying the distal small intestine during medical imaging.
Step-by-step explanation:
The most effective oral contrast agent for opacifying the distal small intestine is barium sulfate.
Barium sulfate is commonly used in medical imaging of the gastrointestinal tract because of its low solubility. When ingested in the form of a suspension, barium sulfate coats the lining of the intestine and appears white on an X-ray image, allowing for greater visual detail than a traditional X-ray.
Unlike the ionic and non-ionic iodinated contrast agents, which are primarily used for other types of imaging studies, barium sulfate is specifically designed to opacify the digestive tract.