Re: Ankle sprain
Buckling when you try to stand on it is not a good sign. That could mean a torn tendon/ligament. Or it could just be painful and feel like it's giving way.
For now, use the boot/crutches, don't push it at all. If you weren't prescribed any medicine, take Advil or Aleve. Keep the foot elevated high (above the heart) whenever possible. Put ice packs on 20 minutes at a time.
Elevating and icing will control swelling, which will reduce pain and allow the tissues to heal. Also, when you do go back to the orthopedist or get an MRI, the less tissue swelling there is, the easier it'll be to examine the foot or read the film.
Give it time. Do not do anything that causes pain.