There are several options,some apply to a back up such as internal medicine categorical at the same time. If they fail to match they have a fallback.Some who dont match and want to repply arrange a research fellowship to be done after internship to make contacts and improve their chances.It may take a year or 2 but many of these individuals do eventually match.It is almost always possible to get internal medicine in the scramble but risky.You may end up in an undesirable program or in a location you dont want to be for three years.The no match rate into Derm is very high,but other competitive fields such as Optho,Rads are much easier to obtain a spot someplace if you apply to abraod range of programs and are not limited by geography.