Why does my mother fixates on one of her nurses and believes that she is poisining her?

It could be anything - just an expanation for why she fels so bad that makes sense to her even though it is not at reasonable, or even she had a nightmare and could not grasp that it did not happen in real life. My mom had vascular dementia and things like this would happen from time to time. Sometimes changing the caregiver or the venue will help, sometimes adjusting meds will help too. Occasionally they even forget about things, but it seems like that idea is pretty well stuck in Mom's head...another word for it is perseveration and it can be part of vascular or Alzheimer's dementia. She'd reason her way out of it if she could - you would think she might realize there is no way you would take the side of a poisoner, or that a facility would let someone work there who was ever caught doing anything like that, but she can't really think things through any more, and so just reacts.

Your welcome! Prior to taking care of my mother I worked in home health. There was patients I had that expressed negative emotions no matter what I did. Alot of times, it was as simple as they was having pain. Instead of saying "I hurt" they would act out in anger. Or they just wanted attention, so they would do everything they could to get under my skin.
Dealing with issues like this has really helped me in taking care of my mom. I have learned instead of getting upset, I need to step back and troubleshoot what is causing these emotions.
I am glad I could help.
Take care :)

Thank you 'yellowfeever', your comment that this could be 'her way of expressing' was like a lightening bolt for me - my mom has aphasia. With everything that's gone wrong with her lately I forget this. It's good to be reminded how high her level of frustration must be. Thanks for keeping me grounded.

I can't offer you a fix it. But maybe if it is only one caregiver, than talk to agency explain its not the caregiver but your moms paranoia. See if the agency is willing to replace this person even if only for a short time. I am not saying that your mom won't displace this paranoia on another nurse. But trying to alleviate your moms negativity would be the best solution.
Alot of times outside caregivers and patients don't mesh well. So since your mom is in a confused state this is her way of expressing that they don't click.
This could possibly be a phase until mom gets well accustomed to this one nurse. What its worth, be forward with nurse that its not her but mom. Maybe she can do things differently to get on your moms "good"side.