Question

phantom pain can occur even when atypical signals from the peripheral nervous system are blocked. so, many scientists think that the central nervous system can contribute to phantom pain.
which step would best help scientists understand how the central nervous system contributes to phantom pain?
scientists could examine the brains only of patients who have not had phantom pain after an amputation.
scientists could study the sensory nerves of patients after they have a limb amputated.
scientists could look for differences in the brains of patients before and after they have amputations.

Explanation:

Looking for differences in the brains of patients before and after amputations would directly show how the central nervous system (brain is part of it) changes in relation to phantom pain. Examining only non - phantom pain patients' brains (first option) doesn't show the contribution to phantom pain. Studying sensory nerves (second option) focuses on peripheral nerves, not the central nervous system.

Answer:

Scientists could look for differences in the brains of patients before and after they have amputations.