If the fitness center caters to people with special medical conditions or an aging population -- as many do -- you'll need to be well-versed in how to handle special circumstances. If you're aware of how to handle the medical condition the employer has mentioned, outline the steps you'd take to ensure the client can exercise safely, including using props or modifying the program, for example.

If you don't remember how to handle that particular situation, don't make something up. Tell the employer that you'd use your exercise science books and consult other trainers for guidance and that you'd be sure to gather all the necessary information before starting to train the client.