Final answer:
A depressant drug may enhance the effect of an inhibitory neurotransmitter, like GABA, leading to reduced central nervous system activity and a calmer mental state.
Step-by-step explanation:
The possible mechanism of a drug that is a depressant could be to enhance the effect of an inhibitory neurotransmitter. Depressants work by decreasing neurotransmission levels, thereby suppressing central nervous system activity, which leads to a reduced level of arousal. One key inhibitory neurotransmitter is GABA (gamma-Aminobutyric acid). Many depressants, such as barbiturates and benzodiazepines, are GABA agonists, meaning they increase the activity of GABA receptors in the brain, leading to a quieter or more subdued mental state. Hence, the correct answer to the stated question is that a depressant may "Enhance the effect of an inhibitory neurotransmitter".