Passage Summary: We are getting really good at swapping out hearts and kidneys to keep people alive, but we can't do that with brains because they have too many nerve connections. Consequently, as we keep people's bodies going longer, we'll see more people with brain diseases that we can't fix with a transplant.

Conclusion: As medical transplants allow people to live longer, the percentage of the population suffering from degenerative brain disorders will inevitably rise.

Reasoning: While many vital organs can be replaced to extend life, the brain's extreme complexity makes transplants impossible, meaning brain health cannot be maintained through the same surgical means.

Analysis: This argument relies on a significant gap between the inability to perform transplants and the inevitability of brain disorders. To make this conclusion work, the author must assume that there are no other medical ways to prevent or treat these brain disorders besides transplantation. We also need to assume that the people being saved by other organ transplants actually live long enough to reach the age where these brain issues typically manifest. Look for an answer choice that identifies a condition necessary for this 'inevitable' increase to occur, such as the lack of alternative treatments.