Sounds quite a lot like my DD (also 10). We've been dealing with anxiety now for > 1 yr - what you said about being fearful of mushrooms, or other poisonous plants is very familiar. Last year DD had a panic attack because my mother bought her a journal with a beautiful metal filigree on the cover. DD liked it but suddenly worried it had lead in it. No amount of logical reasoning at this point made any difference.

My DD is very anxious around health issues, so lots of things can trigger an anxiety reaction - worries about poison, lack of a specific nutrient in her diet, somebody mentioning a disease... it is very stressful and hard to deal with. And as you've noticed, with kids that are very smart and imaginative they will quickly think of loopholes in your explanations, as well as new things to worry about.

Also, reassurance - it is a natural thing to want to reassure a worried child, but with anxiety if you provide reassurance the child will feel better for a short time, but then come back to you again and again for more reassurance. We've found that we need to really minimize this by providing an answer to a question one time only, then remind her to think of what the answer was.

Another thing to keep in mind - when a person is feeling anxious they are not likely to think logically. It is best to wait until they have calmed down to engage their logical minds. Look into techniques your DD can use to calm herself - meditation, breathing exercises, drawing or coloring (there are lots of very nice 'meditative' coloring books out there now), listening to music, and so on.

If you find the anxiety is not improving or is affecting her life (e.g. my DD sometimes refused to go to school when anxious), you may want to consider therapy.

Also, I found this website to be helpful:

http://www.anxietybc.com/parenting/addressing-excessive-reassurance-seeking

http://www.anxietybc.com/parenting/anxiety-101

http://www.anxietybc.com/parenting/talking-your-child-about-anxiety