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When the brain does not receive sufficient blood flow, the most immediate and noticeable symptoms are often related to the brain's need for oxygen and nutrients. Light-headedness is a common response as the brain signals a reduced capacity to function due to insufficient blood supply. This can lead to a sensation of dizziness or the feeling that one might faint, which occurs because the brain is not getting the oxygen it requires for normal activity.

While fatigue and loss of energy can also be associated with decreased blood flow, they are more generalized symptoms that may not be as directly linked to acute blood deficiency in the context of the brain's functioning. The brain's critical area for processing and regulating bodily systems can be affected very quickly, leading to light-headedness and potentially resulting in loss of consciousness.

Thus, the experience of light-headedness and the possibility of passing out is the most immediate and critical response that highlights the consequences of inadequate blood supply to the brain, making it the best answer to the question posed.