Give them an astonished look and say:
"Go to medical school? Are you crazy
?? All that time in school and then be on call? I can get through school a whole lot faster, and when I leave work, I'll leave it behind. Besides that, nurses can change areas pretty easy. If a doctor gets tired of doing cardiology, she can't just go start delivering babies!!"
And if someone says something about bathing people and wiping butts:
"Sure, doctors have so much more fun, looking up noses and doing rectal exams!!"
These are, of course, vast simplifications of the roles and duties, but should be enough for the general population.
And, if someone says something about all the money doctors make, just give them a big smile and say "Well, I'll just have to marry one, won't I??" (That will probably be their next question, anyway!)