Letters

Talk to the PI, get the letter, and let go of the drama.

^ I agree. Why should her words be better than yours?

Remember not to accuse other people--only acquit yourself of any undue blame and argue your actual worth.

You should definitely ask the PI for a rec. If she feel unable to write you a good one, she should decline right off the bat. Otherwise, don't worry about it. She knows you in more ways than the lab (because you've aced her classes) so I see no reason why she would take the grad student's word over yours.

And if you want to give a middle finger to your grad student, just get into med school period. She'll never know what letters you used to get there. Or you could just actually give her the finger.

All good comments above. Another thing you can do is ask the PI to write the letter to demonstrate that you value their opinion of you and the time you've worked together, and then at a later date, decide whether or not you actually want to use the letter. However, some of the options above are probably a better route to go.

-admissions committee interviewer / senior medical student