回复：what is "做fellow"?

In his/her case, doing a fellowship is for further subspecialization training after general internal medicine residency training. It usually takes another 3 years. People tend to subspecialize in specilties require instrumentation/procedures such as bronchoscopy, GI endoscopy, cardiac catheterizations, etc. But neurology, hematology/oncology, rheumatology, endocrinology, and others are also sought. Theose specialties tend to demand high pay and (debatable) better life style -- but that is another topic.

Hope that helps.