The Mega Mushroom is an item in the Mario Kart series. It will make you huge upon usage, also, any Karts you bump into will be squashed. You can also travel off-road without speed loss. It will also relieve you of getting squashed yourself (e.g. getting flattened by a car in Moonview Highway). Although it is impervious to most items, it will still be affected by Bullet Bills, anyone with a Star, and a Thunderbolt (the first two making you flip over like any normal racer, and the Thunderbolt making you return to your original size). It also can lift up a car in Moonview Highway, but it will flip it over like it does with a star.
This item is typically obtained in the middle of the pack.