Using a credo blade or razor to cut calluses on a client's feet is:

Using a credo blade or razor to cut or shave calluses is prohibited in many states and considered unsafe, because it can easily cut the client, cause bleeding, and lead to infection — and such cutting may be outside the cosmetologist's scope. Calluses should instead be smoothed with appropriate files or treatments, not cut with a blade. Knowing that callus-cutting blades are restricted/banned and unsafe is an important pedicure-safety rule. The exam expects candidates to know not to use razors/credo blades on skin.