From the Super Mario Wiki
A Mushroom Block is an item in Super Mario Bros. 2 that is found in Subcon. They are mostly found above ground in castles and World 7. They can be used to defeat some enemies, or stacked up to reach higher areas. They are required to be thrown in order to defeat bosses Tryclyde, Fryguy, and gray Birdos. Mushroom Blocks can also be found in most of the red Birdos' battle rooms. The Mushroom Blocks replace masks used in Yume Kōjō: Doki Doki Panic.
Different worlds have Mushroom Blocks with slightly different patterns, all of which are present in the picture to the left, as follows:
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