From the Super Mario Wiki, the Mario encyclopedia
The Challenge Node is an area found inside Bowser's body in Mario & Luigi: Bowser's Inside Story and its 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. It is, apparently, located in Bowser's spleen. This area is completely optional and does not need to be entered to complete the game.
It is one of the first areas available to Mario and Luigi in Bowser's body after they clear Trash Pit. The first time Mario and Luigi enter the area, and when a new challenge becomes available, the bros. will be greeted and talked to by an Emoglobin. Its icon on the map of Bowser's body looks like an opened Nintendo DS with two dots of eyes on the top screen and one of mouth in the bottom, which the Emoglobins in the area also resemble.
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