Which part of the brain is most affected in deep coma?

ANSWER: (D) RASREF: HARRISON'S 18th ed ch: 274The Anatomy and Physiology of ComaAlmost all instances of diminished alertness can be traced to widespread abnormalities of the cerebral hemispheres or to reduced activity of a special thalamocortical alerting system termed the reticular activating system (RAS).The proper functioning of this system, its ascending projections to the cortex, and the cortex itself are required to maintain alertness and coherence of thought. It follows that the principal causes of coma arelesions that damage the RAS in the upper midbrain or its projections:destruction of large portions of both cerebral hemispheres; andsuppression of reticulocerebral function by drugs, toxins, or metabolic derangementssuch as hypoglycemia, anoxia, uremia, and hepatic failure.The proximity of the RAS to midbrain structures that control pupillary function and eye movements permits clinical localization of the cause of coma in many cases. Pupillary enlargement with loss of light reaction and loss of vertical and adduction movements of the eyes suggests that the lesion is in the upper brainstem. Conversely, preservation of pupillary light reactivity and of eye movements absolves the upper brainstem and indicates that widespread structural lesions or metabolic suppression of the cerebral hemispheres is responsible for coma.