i've been wondering this too. there are many programs out there, and more and more are directed to applicants not just in IM and path (e.g. rads, surg). if you get into a combined program (residency/phd or fellowship/phd) then it's unlikely you'd have to take the gre. if you tried to get the phd separately, then you might be subject to the normal grad school requirment (take gre). obviously the the combined programs are very competitive (many offer admissions to fellowship of choice) since they intend to groom faculty and thus require appropriate pedigree coming in. the apsa webpage is a good place to start looking but i've noticed some are not listed there. n.b., regarding competition, i noticed on the STAR website all the ppl in the derm research pathway were all md/phd already.