Final answer:
The patient should drink barium sulfate oral contrast last for the best visualization of the stomach wall in an X-ray, due to its opaqueness to X-rays and its ability to coat the stomach lining without being absorbed.
Step-by-step explanation:
To best visualize the stomach wall in medical imaging, the patient should drink barium sulfate oral contrast last. Barium sulfate is a contrast medium particularly suitable for X-ray imaging because of its high atomic mass which renders it opaque to X-rays, providing a clear outline of the digestive organs. This "barium milkshake" consists of a suspension of very fine BaSO4 particles in water. When ingested, it coats the stomach lining without being absorbed due to its very low solubility, as indicated by its solubility product (Ksp) of 1.08 × 10−10 at 25°C. Once the barium sulfate coats the stomach wall, it provides greater visual detail than a traditional X-ray, aiding in the diagnosis of conditions such as acid reflux disease, Crohn's disease, and ulcers.