Which of the following best describes the effect of serotonin released by MDMA?

The release of serotonin by MDMA, often referred to as ecstasy, significantly enhances mood and causes increased arousal, making the answer correct. Serotonin is a neurotransmitter associated with feelings of well-being and happiness. When MDMA is consumed, it prompts a massive release of serotonin, leading to the euphoric feelings and emotional closeness that many users report, along with heightened sensory perceptions and energy.

This context explains why the other options do not accurately describe the effects of serotonin released by MDMA. Triggers of sleepiness and increased anxiety are not aligned with the characteristics of serotonin release associated with MDMA; rather, the drug is known for energizing effects and mood enhancement. Furthermore, stating that it has no significant effect contradicts the well-documented impact MDMA has on serotonin levels and, consequently, on mood and behavior.