There are very few monsters or effects in D&D 5e that deal ability score damage or ability drain, but they are various enough for this to come up.
Say you are a 5th-level wizard with an Intelligence score of 18. That means you have 9 spells from your spellbook prepared. If you failed the saving throw against Dyrrn's lair action (description found in the link) and had your Intelligence score reduced by 2, would you then "lose" one of your prepared spells? If yes, how is which spell you lose determined?
