Everyone who is saying, "This is unrealistic because there are donor lists and protocols and.... blah blah" needs to slow down, take a deep breath, and realize that everyone knows this and you're missing the point of the question. If an interviewer asked this, she/he is well aware, and is well aware that you are aware, that it's not a realistic situation. The question is being asked assuming it were a realistic situation. Anyone with half a brain knows docs don't just decide who gets organ transplants on their own ("Well Mr. Jones, sorry, but I went with the other guy instead.")
To use an analogy, it's like answering the question "If you could only have three things on an island for the rest of your life, what would they be?" by saying: "Well, I wouldn't be there for the rest of my life, because I'd build a raft or flag down a cruise ship or..."
In fact, I'd venture to say that an interviewer might even be unimpressed if you answered this question talking about how it isn't realistic, since it'd indicate that you think something that's clearly common knowledge (docs don't just hand out organs as they see fit) is in fact some creative, original insight.