23. The patient needs to have a series of tests of the gastrointestinal system. Which test must be scheduled last? 1. Barium studies 2. Stool sample for ova and parasites 3. Colonoscopy 4. Flat plate of the abdomen

To determine which gastrointestinal test must be scheduled last, we need to consider the nature of each test and how they might affect one another: 1. **Barium studies**: These involve the use of barium contrast material to visualize the gastrointestinal tract through X-rays. Barium can remain in the system for some time and may interfere with other tests. 2. **Stool sample for ova and parasites**: This test requires a stool sample and is generally not affected by other tests. It can be done at any time. 3. **Colonoscopy**: This is an invasive procedure that allows for direct visualization of the colon. It is often performed after barium studies because barium can obscure the view of the colon. 4. **Flat plate of the abdomen**: This is an X-ray of the abdomen that can be done before or after other tests, but it is generally less invasive and does not require special preparation. Given these considerations, the test that must be scheduled last is the **Colonoscopy** (option 3). This is because barium studies should be completed before a colonoscopy to ensure that the colon is clear for proper visualization.