Welcome! It is certainly a Medical Corps pin, I'm guessing for an officer's lapel. I'd have to look around, as it's different than the ones I'm familiar with.
What size is this? And are there any makers marks on the back? (besides "sterling")
The design and type of pinback would lead me to say that it's an Army Medical Corps sweetheart pin. Though I've never seen this particular one, but I've seen sweetheart pins with pretty similar designs (backless corps insignia inside of a circle, with "US" at the bottom). If you do a Google image search for "corps sweetheart pin" you'll get a couple of hits that are close. That's my best guess. Nice find!
Thanks a lot guys. You've put me on a path for a more specific search. You're right Murph; I've found similar items on the Web, but nothing exactly the same. I'll keep searching. I'm trying figure out if it might have been my mom's. (She was a Navy nurse during WWII.) It seems like this style might be pretty rare.
You know, fun fact: Many people associate that symbol with the science of medicine, while in reality it this symbol represents trades, occupations, or undertakings associated with the god Hermes, the messenger god called a caduceus. The true symbol of medicine or health care is the asklepian. If you ever see an ambulance, you will actually see an asklepian on the side.