The term ‘registered nurse’ is not considered a proper noun. Proper nouns are specific names of people, places, or organizations, such as ‘John’, ‘London’, or ‘NASA’. However, ‘registered nurse’ is a common noun that describes a type of healthcare professional who has completed the necessary education and licensing to practice nursing.
When referring to a specific registered nurse by name, such as ‘Registered Nurse Sarah Johnson’, the title becomes part of a proper noun in that context, but on its own, ‘registered nurse’ remains a common term used to denote the profession and does not capitalize unless it starts a sentence.