The key here is that there are few spots nationally. The University of Pennsylvania will also allow you to get your PhD in a diverse choice of departments, including at least one student here in Psychology, however this is the exception--not the rule. The vast majority of students at any fully-funded MD/PhD program are in basic science, and there are usually a handful outside of that at the few programs that will allow it, including at the University of Illinois.
What's important is for the so far unresponsive op to really consider whether he or she wants to fight the uphill battle to get one of these spots.