Final answer:
The priority rationale in designating a member of the family as a healthcare surrogate is the individual's familiarity with the client's complex medical history.
Step-by-step explanation:
The priority rationale in designating a member of the family as a healthcare surrogate is the individual's familiarity with the client's complex medical history. This is because the healthcare surrogate will need to make informed decisions on behalf of the client, and having knowledge of the client's medical history will enable them to make the most appropriate decisions that align with the client's wishes. While other factors like the individual's knowledge about the client's intended decisions and their acceptance of death or disability may also be important, the familiarity with the client's medical history takes precedence in ensuring effective healthcare surrogate decision-making.