Can i srain my ankle by falling down a flight of stairs and hitting it with my clarinet case

Oh yes!: You could have sustained a number of injuries by falling down a flight of stairs, ranging from an ankle sprain to a broken bone or head injury. This probably happened so fast, you may not have realized immediately what you injured. I hope your clarinet didn't get hurt. Unlike it does for an ankle sprain--Ice, an ace wrap and a few days off of its feet--wouldn't have made it better.