Geriatrics is actually a subspecialty of Family Practice, as is Sports Medicine. Geriatrics and IM differ in that the patient population tends to be older in Geriatrics, and there's a lot more psychological stuff that goes along with being a geriatrician. Internists deal with chronic illness, and that's why many of the patients tend to be middle-aged. Diseases like diabetes, emphysema, hypertension, and other chronic conditions, are handled best by internists. Geriatricians also deal with chronic illnesses, but have a fountain of knowledge when it comes to old-people problems: particularly with respect to management of infections, end-of-life care, etc.
Primary care is IM, FP, Peds, Psych, and SOMETIMES OB/GYN. Some consider OB to be a little bit above the primary care programs.
There are three different IM programs available. There's an IM Categorical, IM Preliminary, and IM Primary Care program. The difference between the Categorical and Prelim programs is that Prelim programs only last a year, and you have to find something to do after that. Categorical programs are the way to go for subspecialization (Renal, Cardio, Resp, etc.). Primary care programs are very focused on ambulatory care (outpatient clinic type of stuff), but I'm not sure if you can subspecialize.
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Tim Wu.