I often appeal to the forced organ donation hypothetical.  See for example my common-sense case for pacifism.  But what precisely is the hypothetical?  Here‘s an excellent explanation, courtesy of Judith Jarvis Thomson:

[I]magine yourself to be a surgeon, a truly great surgeon. Among other things you do, you transplant organs, and you are such a great surgeon that the organs you transplant always take. At the moment you have five patients who need organs. Two need one lung each, two need a kidney each, and the fifth needs a heart. If they do not get those organs today, they will all die; if you find organs for them today, you can transplant the organs and they will all live. But where to find the lungs, the kidneys, and the heart? The time is almost up when a report is brought to you that a young man who has just come into your clinic for his yearly check-up has exactly the right blood-type, and is in excellent health. Lo, you have a possible donor. All you need do is cut him up and distribute his parts among the five who need them. You ask, but he says, “Sorry. I deeply sympathize, but no.”

Would it be morally permissible for you to operate anyway? Everybody to whom I have put this second hypothetical case says, No, it would not be morally permissible for you to proceed.

P.S. Thomson credits Phillipa Foot, but the latter’s discussion is fairly cursory:

Why then do we not feel justified in killing people in the interests of cancer research or to obtain, let us say, spare parts for grafting on to those who need them? We can suppose, similarly, that several dangerously ill people can be saved only if we kill a certain individual and make a serum from his dead body. (These examples are not over-fanciful considering present controversies about prolonging the life of mortally ill patients whose eyes or kidneys are to be used for others.) Why cannot we argue from the case of the scarce drug to that of the body needed for medical purposes?