From the Super Mario Wiki
Gut Check is an area inside of Bowser's body in Mario & Luigi: Bowser's Inside Story. It is apparently located in Bowser's stomach. When the Wiggler challenges Bowser to eat the giant carrot, Mario, Luigi, and Starlow come here to help him digest the carrot. The player must tap on each piece of carrot three times to make it disappear, and when all the carrot bits are gone, Bowser will eat more. There are also special enzymes that help the player get rid of the pieces of carrot faster. The player must finish the entire carrot in less than one minute to proceed. If the player doesn't eat the giant carrot the first time in under a minute, then the Wiggler will go out and replace the giant carrot with an equal amount of regular carrots with the same rules.
Also, Mario and Luigi come here a second time to get the second Star Cure after Bowser swallows it.
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