he word "doctor" is a noun because it represents a person. Nouns are used to name people, places, things, or ideas. "Treats" is a verb showing an action, "in" is a preposition, and "sick" is an adjective modifying the noun "patients." Other nouns in the sentence include "patients" and "hospital," but since the question asks for one noun, "doctor" is the correct answer. Identifying nouns helps in structuring grammatically correct sentences.