> is it morally wrong for the secretary in a medical office to be paid less than what the physician owner takes home in salary/dividends?

To the extent that their differing roles are circumstances of birth and life, and that fairness is part of your morality, yes, it is.

> Do they have teenage pregnancy? Do they study hard in high school or trade school or professional school or engineering school? Do they have business intuition to start their own business?

Those are all circumstances of birth, be it nature or nurture.

We have a strange notion of "deserve" in our society, that someone deserves more simply because they have a skill or circumstances that are in lower supply compared to demand. So physician skills are in shorter supply than secretary skills. But if they were shipwrecked on a desert island, and the secretary knows how to fish and keep them alive, suddenly he can command a greater share of income on that island. Arbitrary no?