Said the philosopher Philippa Foot, who has just died at the age of 90. (She's one of those people, like, supposedly, Shakespeare, who died on their birthday.)
Foot originated the famous trolley problem (in an essay called "The Problem of Abortion and the Doctrine of the Double Effect"). In the hypothetical, a runaway trolley continuing along the rails will hit 5 people who are working on the track. Should you actively divert the trolley onto a track where it will proceed to hit just one person? If you say yes, then should we favor killing a person to get 5 organs that can be used to save 5 lives?
Here's a PDF of the essay.