It's always "kosher" to write accurately and concisely about your past in primary essays, including the research essays. If you were very interested in a particular area of focus, or a disease, then the right thing to do when recapitulating your experiences in the essay is to discuss that interest. In many ways having a unifying thread can be a powerful way to structure an application package.
However, in my experience, adcoms and faculty like to stress that interests change over the first few years of med school and grad school, when you are exposed to a variety of different biomedical topics. Also, PIs move, have variable funding, change focus, etc, and so the blanket advice for MD/PhD applicants is to find an institution with 3-5 PIs who you would be very interested to do research with, instead of attending X institution because of Y's work on Z disease. So, you may be writing yourself into a corner if your app sells you as a "Parkinson's guy" for example; you limit the potential of the institution to leverage your skills broadly, and you are effectively limiting your own exposure to other areas of research interest.
One way to cover your bases in the primary is to talk about how interested you currently are in this disease, and how you have organized your undergrad experience around developing a depth of knowledge in that area, and that you'd like to continue to delve deep into those sorts of problems during your PhD -- but, that you are open to tackling new problems (within a certain overarching field which you should specify) and that the breadth of expertise at an institution is what draws you to work there. I think it is best to place a short statement about this in the primary but to expand on it considerably in the secondary, where you can list specific faculty or research groups that fuel your interest in that program.
A last point -- some of the most amazing MD/PhD thesis projects I have seen have been interdisciplinary. In almost every case, when I asked the student in question how they developed their project, the answer was that they had learned something from med or grad classes, or had stumbled upon a different faculty member working in an unrelated field, and they saw a link between their area of interest and the other. Science is only artificially compartmentalized and you may find that the best way to tackle studying X is actually to use Y technique, or look at Z molecule, etc.
