Taking the Big Mushroom makes your character larger! The max HP is worth the draw down if you're close to the end of the run, since it buys you more time in your boss fight. Random upgrades are decent in Act 3 since you've likely removed a lot of starter cards by this point. If you can't afford losing turn 1 draw for any reason or you don't need more hp, then option 2 is a solid pick.
How long has it been since you ate?
...wait, what's that fantastic smell?
Following the scent, you reach a cozy campground with all manner of tasty mushrooms being cooked! You don't consider the safety of eating these mushrooms because you are so hungry.
(So hungry that you don't notice the dead adventurer)
Big Mushroom
You bite into the Big Mushroom. Its flesh is firm, starchy, and satisfying. The more you eat, the hungrier you become, as if the mushroom is feeding on your hunger.
So yummy... you enter a food coma.
This gluttony will cost you.
Fragrant Mushroom
You sample the Fragrant One, a small woody mushroom with a delicate scent of shellfish...
A burst of energy spurs you to run, jump, and rigorously train!
Then, a sudden stabbing pain hits you as the living fungi struggles within you during its brief final moments.
Medley
The medley of mushrooms looks the most appetizing so you go for that.
“MY MUSHROOMS!!”
The adventurer, presumably dead, attacks you! You knock them out, and with a seasoning of guilt, finish the rest of the fine meal.