The Mega Mushroom is an item that only appears in Mario Kart Wii. It is most often acquired in middling positions (4th to 9th place). This item makes the racer super-sized. Lightning bolts can affect them, turning them back to normal; they also cannot knock trucks up in the sky (in Moonview Highway). If two racers have used a Mega Mushroom, but one also uses a Star, then the racer without a Star will also get squished.
The Mega Mushroom was originally going to reappear in Mario Kart 7, but it was scrapped due to incomplete functions. Its codes are still in the game's data. In Mario Kart 8 and its Nintendo Switch port, it didn't appear as and item at all.
The Mega Mushroom is mainly an offensive Item. Once a racer uses it, they become extremely huge. While super-sized, the driver can squish any racer in front of them (similar to the vehicles in Moonview Highway, although you are only squashed for a shorter amount of time compared to the Mega Mushroom, and also similar to Thwomps, except you only get squished if you get squished, and they move in one spot.). This slows down the flattened opponent, and it takes them a while to regain there full size. Also, players are impervious to any other Items, excluding the Bullet Bill and Star. If a driver is behind you with the Mega Mushroom, a warning alarm will appear (Usually, it is too late. So don't bother. Just enjoy your last moments before you are squished.).
In other games
The Mega Mushroom made its debut in New Super Mario Bros.. It makes Mario or Luigi almost the size of the screen and able to destroy almost everything on screen by running into it. The music for when the Mega Mushroom is in use is used in Mario Kart Wii.