This Feature is Currently No Longer Implemented in the Game
Consumable Quest Items are items that are consumed when doing certain quests.
There was only one Consumable Quest Item: the Skull Key. You could get one by doing the first three quests in the Undead Realm, and they were needed to do the other two regular quests in that area (in lieu of any other requirement). You consumed 1 key every time you did a quest that required you to have the Skull Key. Considering that these keys used to drop very infrequently, you ended up doing the quests that drop Skull Keys a whole lot more often than any other quest (if your goal was to complete the Level 4 challenge). To see how many Skull Keys you had, they were tracked on the Alchemy page.