Hi -- here is a reply to the anonymous correspondent who emailed me the original post in a PM.
The person who replied mistakenly thought I was the one posing the question (I was just the messenger). His reply is very nuanced so I am taking the liberty of posting it below.
I'm sure the original PMer will appreciate the reply, and I am glad that so many residents are interested in commenting and contributing.
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My story sounded much like yours so perhaps my experience can be helpful in answering some of your questions.
In summary I, too, did my graduate work in a quite non-derm-related field. I initially actually thought I might want to do Cardiology given my research but also wanted to go into the clinical years of med school with an open mind. One feature of my PhD years that helped to "steer" me was that my project required a lot of surgical prep, which I loved. Also, I ended up actually liking to do research and wanted to choose a field that would allow me to spend time with patients and with whatever my research interests ended up to be. I kept Dermatology in the back of my mind throughout grad school because I had really enjoyed the "skin" parts of the preclinical med school lectures and because I already knew that it was a specialty that might allow me to relatively more easily combine clinical and research time. Enough about me, though, let's try to answer your questions:
1) How can I find more about about Derm and if I can develop a passion for it?
My school allowed students to postpone one required rotation in favor of two electives during third year of med school. I did this and took Gen Derm and Peds Derm. I made an effort on both rotations to start working on (small) manuscripts. We're talking case reports here. I also arranged to meet with the chairman of the derm department during my Gen Derm rotation. Hence, I had NO contact with Derm until the second half of my third year of med school, which is also when I realized that Derm is what I wanted to do.
2) During my PhD (as in now) should I be trying to find a Derm mentor? If I don't know a lick about the Practice of Derm, how can I get my foot in the door? Should I wait until I start third year?
See my answer above for the answers to this. In my experience, having done the PhD work kind of cancels out some of the bright-eyed-and-bushy-tailed gunning that "regular" med students have to do to get in the Derm door. There may be relatively more MD/PhDs applying for Derm, but even so they still have an advantage in most Derm programs over "regular" med students.
3) How can one be a competitive candidate?
Board scores. Grades during third/fourth year. Dean's letter. Given that you're an MD/PhD, you should be expected to have a more prolific cv than a "regular" med student. I honestly got the sense on the interview trail that quantity of publications was more impressive than quality. Which is why I had no problem churning out several case reports during third year of med school (NONE of which were published by time interview season hit) rather than focus all my efforts on one project that may or may not be completed.
4) How does one get the attention of the Chair for a good LOR? Other places to get good LOR?
As in #1, I scheduled a meeting with the chairman during my first Derm rotation. Things may have been a bit easier at my program because the chairman also has quite a bit of interaction with students, so he subsequently worked directly with me and could speak about my clinical and my research experience with him. My LOR for derm were: my PhD mentor, the chairman, another dermatologist from my institution that is well published (and well respected in the field), and one of my attendings from my IM rotation during third year. I had absolutely no trouble with these LOR.
One other thing that I think is important for you to know: I only rarely encountered people who expressed skepticism about how I would translate my grad school work to dermatology. When asked, I simply said that I felt that the most important part of grad school wasn't the project, but learning how to DO research (how to write papers, obtain grants, formulate workable hypotheses, etc), and I felt like I got a very good foundation in that. That seemed to satisfy just about everyone.
I ended up with 17 interviews and matched at my #1 program. The PhD will help you.