Final answer:
Fecal tagging is not a preparation step for CT colonography; it's part of the interpretive process, whereas colon cleansing, rectal administration of barium, and colon distention are essential preparations for the exam.
Step-by-step explanation:
The procedure that should NOT be done in preparation for a CT colonography exam is fecal tagging. This is because fecal tagging involves the intentional labeling of residual fecal material in the colon to distinguish it from polyps during the CT colonography, which is not a preparation step but rather a component of the interpretive process. The other steps listed, such as colon cleansing, rectal administration of barium, and colon distention, are essential preparations for a CT colonography. Colon cleansing is necessary to ensure the colon is clear of any waste that could obscure the view of the colon's interior, barium or a similar contrast agent is used for coating the lining of the colon to improve image quality, and colon distention by air or carbon dioxide is critical for a better visual representation of the colon's structure.