sorry to laugh but i have been there. It is usually fear the only thing i found that even works a little Book the exam 15 minutes before the vet comes in open the carrier but dont let her out. Have the vet put a scent on her hands that poppy likes, cat nip if she is into that. When the vet comes in, ignore poppy for a few minutes and have a conversation with the vet in a nice calm manner.
Of course dealing with cats this may also be a complete waste of time. At least she isn't as bad as one of mine i had who would sink her teeth into the vet at first chance and never got out of doing that.