Psychoactive drugs have various mechanisms through which they affect the brain, leading to a wide range of physiological and psychological effects. Understanding how different classes of drugs function is crucial for both psychological study and real-world applications. For instance, the impact of a specific drug can depend not just on its class but also on individual biological factors and contextual elements. In particular, consider an antagonist that is designed to block a specific neurotransmitter's effects. Which of the following drugs acts primarily as an antagonist, inhibiting the action of a neurotransmitter in the brain?