Lump of brain

 is an item that the Lone Wanderer finds in the Point Lookout add-on for Fallout 3. It is a piece of the Wanderer's own brain in a glass jar.

Location
The item is in the Engine Room of the Duchess Gambit, on the table next to Tobar. You can only get in this room after completing the quest Walking with Spirits and speaking to Nadine in the Ark & Dove Cathedral. Once obtained, it can be used as a great decoration for the player's house/suite/flying saucer.

As mentioned above it can only be obtained after talking with Nadine, but also only after you've asked her to remove your scar; you then have the option to ask her why the Tribals would cut your head open. This effectively makes you unable to obtain The Lump of Brain and keep your scar, without using console commands.