Which of the following is a procedure used by veterinarians to administer certain medications or to remove toxins by flushing the lower intestinal tract?

The procedure that veterinarians use to administer certain medications or to remove toxins by flushing the lower intestinal tract is an enema. This method involves introducing fluid into the rectum, which can help to cleanse the bowel and allow for the administration of medications directly into the lower gastrointestinal tract. In many cases, this is especially useful for treating constipation or for preparing the intestinal tract before diagnostic procedures or surgeries.

Other procedures like colonoscopy and endoscopy are more invasive diagnostic techniques that visualized the interior of the gastrointestinal tract but do not typically involve the flushing and administration of medications in the same direct manner as an enema. Intravenous therapy involves delivering fluids or medications directly into the bloodstream and does not pertain to the lower intestinal tract at all.