Hello. Thank you for coming to JustAnswer with your question, and welcome.
As I think you understand, this is very difficult to do. Realistically, you need to approach the her when she is "in the mood" to take medications. It sounds like she has a bit of dementia, or at least a delirium caused by her illness and being in the hospital. You will not be able to reason with her when she is in this state.
If she is not in the right state of mind to take her meds willingly, the only way to get them in would be to force them on her (which is unethical in most cases). You can try to entice her with some ice cream or apple sauce with the medications hidden in it, however if she realizes what you are doing it could make the situation worse, and create a situation where she is even more distrustful of you and/or medical personnel. Unfortunately, the more tricky you get, the more stubborn and paranoid the patient gets.
The best thing to do is to let the physician know that the prep cannot be completed as she is refusing to take any pills by mouth. There may be another option (a liquid perhaps you could mix into something). Other options might be a different type of test, or even rescheduling the test so you can try again another time.
Your description of her symptoms and behavior is classic. Her cognitive and emotional state has been negatively influenced by her illness, change of people around her, change of environment, along with all the "procedures" that are being done to her and around her---none of which she able to understand. So, this creates fear and paranoia. Any time she responds to something (anything) with such behavior, the best thing to do is to back off. Reassure her. Let her relax for awhile. Then re-approach later.
Hopefully the hospital staff can complete her treatment as soon as possible so that she can return to more familiar surroundings and routines. This will help a great deal.
The other thing to consider might be the actual importance of the test. Ask yourself if any findings are going to change her treatment plan to improve her life. If not, maybe it really doesn't need to be done. Just a thought.
At this point, the primary goal (I would think) would be her comfort, be that physical, emotional, or cognitively.
Good luck to you. And best wishes for your Mom's recovery.
You're welcome, and thank you.
(note: I must post this as an answer, but do not feel obligated to accept again).