What is the difference?

I have no idea, I never heard of patient access

I don't know what either of the are, let along what the difference is. Sorry!

Never heard of these terms sorry

Medical Assistant is the person you see before the doctor. They typically do the blood pressure, tempature , breathing, etc. They take down the list of symptoms you have. They also do blood draws and give the shots and help with office procedures like removing a simple wart to helping place a brace.
I went to school for MA and have a license for it but I am not currently doing that kind of work.

This what I found for patient access rep

Coordinate communication between patients, family members, medical staff, administrative staff, or regulatory agencies.
Interview patients or their representatives to identify problems relating to care.
Maintain knowledge of community services and resources available to patients.
Refer patients to appropriate health care services or resources.
Investigate and direct patient inquiries or complaints to appropriate medical staff members and follow up to ensure satisfactory resolution.

It doesn't look like a patient access rep helps with patient prep when they're waiting to be seen by the doctor. You can learn more about the exact job description of medical assistants at the BLS website