Fungus Deal

 refers to an unmarked and repeatable quest in Fallout 3.

Talking with either Lucy, or Eclair begins this quest.

Quick Walkthrough
1. Talk to Lucy and Eclair.

2. Get permission to trade from Mayor MacCready.

3. Trade Buffout and strange meat for cave fungus.

Walkthrough
Lucy states she needs Buffout, from which she can derive a useful treatment for the rickets. Little Lamplighters are subject to malnutrition due to lack of exposure to the sun.

Eclair, Little Lamplight's cook, reveals that he is in need of strange meat, which is actually human flesh. Eclair explains that there is a fungus that grows in the cavern's pools and makes up an important part of the children's food supply. This fungus seems to thrive on meat, especially strange meat, a discovery the first children of Little Lamplight made when they threw the bodies of dead adults into the pools.

The player must get Mayor MacCready to agree to the exchange deal. It appears the player requires about 50 in barter or Child at Heart perk to make him agree. If you decide to do a speech check (which is still possible after doing the Barter check) and succeed, you will threaten MacReady into giving you 10 Cave Fungus. However, Eclair will no longer trade Cave Fungus with you, as well as sell you his other goods (he will instead say "We're cut off. Go scrounge your own food."), and Lucy will act as if you never negotiated with MacReady. Doing the speech challenge is ill-advised.

With MacCready's approval, Eclair and Lucy will both trade one unit of cave fungus for three pieces of strange meat or three Buffout.
 * With the Child at Heart perk it will be one piece of strange meat or Buffout for one piece of fungus. It should be noted that having a higher barter at the time of the agreement will lower the number of Strange Meats/Buffout per fungus. A value of 75 barter will net a two-for-one deal. It is possible to renegotiate the terms of the deal by returning to MacCready with a higher barter skill later.