Your best friend
Use what you have
Steal some from a nearby Clan
Tell him you have a plan to fake it, and feed him juniper berries instead
Feed him deathberries anyway; he's no loss, and you wouldn't dare to anything to threated your medicine cat role.
Think, 'We can find him a place in the Elder's den. Our leader wouldn't dare harm any cat.'
Wail like a kit, "Oh NOOOO! What can I do NOW? Will my leader make me kill it?"
Go off with him anyway; it's not like anyone will know, will they?
Talk to your old mentor anyway; he/she might be able to help you.
Snap at it, and tell it to go away right now.
Calmly tell it to go give it to the nursing queen, and that you're busy right now and not hungry.