Here, 'this rite of passage' refers specifically to the MCAT , a necessary step on the way to becoming a doctor.
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jooneydefined this rite of passage into the medical profession,I think you might be parsing this wrong. "into" doesn't go with "defined"; it goes with "passage". It's not that something is "defined into" something else. It's that something defines/characterizes (the passage of one thing into another thing). Thus, the quoted part means