Great piece!
Meet Brian, he's a pretty nice guy. I met him in my general biology during my post-bacc. He was a freshman at the time, and was his first science class. I sat next to him in class, and we became friendly. He was a biology major, and had aspirations to work as a cardio thoracic surgeon. Today he works an EMT (he did not work as an EMT during undergrad). Brian was for most part following the linear path of what most pre-meds do. He was a biology major, started volunteering once weekly in a hospital, and was studying. Unfortunately for him, biology started off kind of rough. He ended up with a D in the class, and was relieved that he passed. At this point of the game (or even earlier), lots of college students would call it quits with pre-med. They would pick up other majors, and move on with their lives. Brian continued with the pre-med courses, volunteering, and shadowed some physicians along the way. He took the MCAT (never told me his score), but I'm assuming his grades didn't allow him to get in. During his senior year, he became a research assistant and then became an EMT after graduation. To this day, he is an EMT and not in medical school.
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The takeaway message: Knowing when to quit is important. The further you linger in the pre-med process, the more doors you shut on reasonable future careers. If you get your ass absolutely handed to you in your first science class and you can't seem to handle the material, get out now! In fact, you should have gotten out yesterday.
I think in this specific case your advice of quitting is warranted, especially if someone works reasonably hard and gets a D at the end of the class, but I think it's important to also emphasize when not to quit and to instead learn to improve.
I've raised many C's/B's to A's over the course of the semester. Also I had two buddies both raise C's to serviceable B+'s in organic as well. A lot of this was through addressing test-anxiety, doing practice problems under timed conditions, and visiting tutoring.
Another misstep I'd say is quitting too early, when the potential for better problem solving and study habits are still there. A C or low B after exam 1 ain't necessarily fatal and may be addressed if you work on yourself as a student.