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Brain death is defined as the irreversible loss of all functions of the brain, including the brainstem, which is responsible for many critical reflexes and functions such as breathing and heart rate control. The determination of brain death requires that there be no activity in the entire central nervous system, indicating that neural function has permanently ceased.

While the absence of heartbeat, unresponsiveness to stimuli, and inability to breathe without assistance can be associated with various medical conditions, they do not capture the complete and irreversible cessation of brain activity. For instance, a person can have a heartbeat and still be in a coma or vegetative state, where the brain is not functioning normally but is not definitively declared dead. The key factor in the definition of brain death is the irreversible loss of all brain activity, which is clearly this option's focus, ensuring a precise understanding of when a person is no longer living, according to neurological criteria.