The Best Medicine, by zaptiftun
Sad

While looking for Pinkie Pie to prank her, Rainbow Dash follows her into a hospital and hears some unsettling things about her friend.

Bring tissues, and make no promises not to cry. Hope you like sad, because here it is.

(I should note: if you haven't read The Light in the Darkness, read that before this. It's fine if you don't; it's just that there's a thing Rainbow says in The Best Medicine that refers to the events of TLitD.)