You don't always have to have an undergrad. A good friend's Dad earned an MD/PhD from the University of Chicago without an undergrad. This was in the 1960s though and he entered after doing a stint in the Peace Corps.
This is less likely now since competition is so high so undergrad courses/GPA is a filter, but it does speak about how an undergrad degree is strictly orthogonal to the job. You could probably get away with it today based on the MCAT as proof of pre-qualification maybe...