As a physician, I can tell you that every US medical student I've seen had to do/learn all the basic proctology tasks/diagnoses, and residents must learn the entire general range of proctology tasks/diagnoses. While most schools don't let a student do, say, full hands-on supervised colonoscopies for liability/inexperience/billing reasons, their residency will expect them to. A proctologist (as you term a board certified internist, with further training leading to a board subspecialty as a gastroenterologist) is an expert, there are no "proctology interns".
I say this as someone who feels US medical care suffers from our excessive (sub)specialization, at the expense of trained generalists.
As abusive as I feel the med school/residency system is, this is one part I agree with: any physician SHOULD have a thorough grounding.