Which could not legally act as an agent for another patient?

A minor cannot legally act as an agent for another patient because they are legally considered incompetent and therefore cannot consent on behalf of someone else. Option B, a parent for their minor child, is incorrect because although a minor cannot act as an agent for someone else, a parent is legally able to make decisions on behalf of their minor child. Similarly, option C, a lawyer for their client, is incorrect because a lawyer is also able to make decisions on behalf of their client as long as they have been appointed as their legal representative. Option D, a spouse for their partner, is incorrect because a spouse is also able to act as an agent for their partner if they have been given the legal authority to do so through a power of attorney. However, a minor is not able to give their younger sister the legal authority to act as their agent.