* Pronouns are words that replace nouns. Examples include: he, she, it, they, you, we, I, etc.
The sentence "The dentist said Open wide" uses the imperative mood, which is used to give commands or make requests. The word "Open" is a verb in this context, not a pronoun.
To make a pronoun work in this sentence, you'd need to include a noun that the pronoun could replace. For example:
* "The dentist said to *you*, 'Open wide.'"
* "The dentist said to *the patient*, 'Open wide.'"