What key information must be included on a client consultation form?

The key information that must be included on a client consultation form is the client's medical history and any allergies. This information is crucial because it helps ensure the safety and well-being of the client during nail services. Understanding any medical conditions or allergies allows the nail technician to make informed decisions about the products and techniques used, thereby minimizing the risk of adverse reactions.

In the context of a nail service, knowing a client’s allergies can prevent the use of products that might trigger reactions, which can range from mild irritations to more severe health issues. Furthermore, having this information fosters a professional relationship built on trust, as clients are more likely to return when they feel that their health and preferences are prioritized.

The other options, while they may enhance the client experience or give insight into personal preferences, do not signify a critical need for safety or health considerations as medical history does. Preferences for nail colors, contact with previous nail technicians, or favorite nail art designs are less essential to the ensuring client safety during the services provided.