What is up with people constantly trying to hide things on their applications? I understand trying to produce the best application possible, but a lot of these things just seem plain dishonest. Trying to hide things like accommodations, DWI, and auditing a course span a considerable range, but they are still things that are likely relevant to your time spent in undergrad.
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Why do you want to audit the class? Why do you feel so ashamed about it that you don't want schools to know that you did it?
In the end.... ask the professor if its okay if you listen to his lectures and just show up. No reasonable undergrad professor would ever turn away someone from his lectures. Given the size of this kind of class, it may even be appropriate to just show up and sit in the back. Or you could just audit the course because its not going to make a difference on your medical school application.
Why not instead of trying to deceive admissions committees you spend your hours doing something to put on your application that people will ask you about and hey, maybe you will learn from and make you a better applicant and future physician?