Here's why:
* Adjectives describe nouns.
* In this sentence, "poisonous" describes the noun "substance". It tells us what kind of substance it is.
Let's break down the other words:
* dangerous and foolish are adjectives, but they describe the action of "leaving".
* unmarked is an adjective, but it describes the "substance" in terms of its lack of a mark.
* shelf is a noun.
Therefore, the only word directly describing the noun "substance" as an adjective is poisonous.