Final answer:
The decreased central nervous system efficiency in elderly patients affecting drug response is primarily related to reduced brain mass, which leads to altered drug receptor sensitivity and changed pharmacodynamics.
Step-by-step explanation:
The decreased efficiency of the central nervous system (CNS) in elderly patients affecting the response to drugs is primarily related to Reduced brain mass (C). This occurs due to a natural decline in the number of brain cells as people age. This reduction in brain mass can lead to changes in drug receptor sensitivity and pharmacodynamics, which may require adjustments in drug dosing or the selection of different medications for elderly patients. Other factors that contribute to the changed response to drugs in the elderly include impaired kidney function, which can affect drug clearance, and decreased digestion ability, which can influence drug absorption.