Which of the following is an example of a Schedule I substance?

LSD, or lysergic acid diethylamide, is classified as a Schedule I substance under the Controlled Substances Act. Schedule I substances are characterized by having a high potential for abuse, no accepted medical use in treatment in the United States, and a lack of accepted safety for use under medical supervision. LSD fits this definition as it is considered to have a significant potential for abuse and is not recognized for any medicinal purposes by the FDA.

In contrast, the other substances listed have medical uses or recognized therapeutic applications. Hydrocodone is a prescription opioid used to treat pain, Valium (diazepam) is a benzodiazepine used for anxiety and muscle relaxation, and Phentermine is a medication used for weight loss. These classifications place them in different schedules, where their potential for abuse and medical acceptance vary significantly from that of LSD.