dont most doctors and dentists come into the service as O-3s, not much of a jump to O-4...at a base I was stationed at in Texas back in the late 60s, it was said one of our dentists, at the time, was the youngest Captain in the Air Force, 22 years of age....
My Father was a medic in WW II. he was a Tech Sargent(just above Buck Sargent). Most of the officers who performed Operations on the Wounded in the field and also performed other functions associated with their status as an MD were Lieutenants.
It is a shame that we are not able to get the qualified MD personnel in the Military without giving them high ranking levels. A Battalion Commander has responsibility for 1000 men and women in combat.