I agree with Dermione above, but I saw this from the AAMC (I don't think any residency program actually believes it though bc it doesn't make intuitive sense, and there are derm applicants who do a research fellowship after their prelim year and match, or match during their internship year, so just don't say anything on your interviews):
http://umsc.org.uic.edu/documents/medicaregme2013.pdf (page 6-7)
Residencies that require the completion of a broad-based clinical year:
• If a specialty requires a broad-based clinical year of training, and you match simultaneously into both the broad-based year and the specialty program, then your IRP is determined by the specialty program that begins during your second year of training.
If, instead, you initially match only into a clinical base year or preliminary year program, your IRP is determined by your clinical base year program—even if you later match into a different specialty.
Here’s an example: You simultaneously match into both an internal medicine clinical base-year program and a radiology training program. Your IRP will be based on the minimum number of years required to become board eligible in radiology and will be set at five years. If, instead, you match only into an internal medicine clinical base-year program, begin the program, and later are accepted into a radiology program for your post-graduate year-2 (PGY-2), your IRP will be based on the number of years required to become board eligible in internal medicine, and will be set at three years.
• If you match into a program that would begin in your PGY-2, and you are able to obtain a preliminary year position for your PGY-1 outside of the match, then your IRP is determined by the specialty in which you will train during your PGY-2.
When your first residency is a transitional year:
• If the first residency you enter is a transitional year, then your IRP is determined by the residency you enter in your second year of training