I don't see why academic med isn't a field. I'd guess at this point most people, if they were being honest, would have to be very general e.g. some kind of surgery, family practice, peds, int'l med. I think academic med is perfectly legit. The only possible negative question I could imagine an adcomm asking is: why don't you apply MD/PhD if you want to do straight research? But since many academic docs just have MD, I don't know if this would be an issue at all. Besides, for Hopkins, I'm sure many of their grads do go into academic rather than clinical practices.
One can be in "academic medicine" in any field of medicine (from family practice to neurosurgery).
In general, you might think of the big fields as: Internal Medicine and the medical subspecialties - Surgery and the surgical subspecialty - Pediatrics - Psychiatry & Neurology. OB/GYN includes surgery but also has a lot more "Primary care" than most surgical specialties. If you can see yourself fitting into one of those broad categories, I think it is ok to say so but with the understanding that you haven't narrowed down your interest.
Then there are the outliers like Diagnostic Radiology/Pathology/Radiation Oncology/ Internentional Radiology (sticking needles and tubes into odd places in the body guide by images on a screen - for all you kids who excelled at video games), Anesthesiology, and Derm. You'd have to make a pretty strong argument, with lots of specific experience, to make a case for that being a particular interest at this step in the process.
If you aren't enamored by surgery and don't feel pulled to care for sick kids or deal with women's reproductive health issues, and don't feel particularly attracted to any one thing over another it is absolutely understandable to say "not sure".