If a product is meant for use by a specific skillset, what depth of knowlege should a designer have in that skillset?

For example, if someone is designing an interface for a product used by doctors for a specific task, the designer will have far less medical knowledge than the doctors using the product. How much will this gap in knowledge affect the design process, and would bringing in doctors (or whoever may be an expert in the field being designed for) to help with the design process to deter this effect? Should the designer research enough to be able to clearly explain what's actually happening when the product is being used, or are they mainly focused on how the doctor interacts with it?