Which of the following substances is a derivative of barbituric acid?

Barbituric acid is a compound that serves as the structural foundation for a group of medications known as barbiturates, which are central nervous system depressants. Nembutal, or pentobarbital, is specifically synthesized from barbituric acid and is used primarily for its sedative and anesthetic properties. This substance retains the crucial barbituric acid core and has additional side chains that enhance its pharmacological effects, leading to its application in various medical contexts.

In contrast, Valium (diazepam) is a benzodiazepine, aspirin is a salicylate used as an analgesic and anti-inflammatory, and meprobamate is a non-barbiturate anxiolytic. Each of these compounds has distinct structural properties and does not share the barbituric acid framework that defines the barbiturates, thereby underscoring the unique position of Nembutal among the listed choices as a direct derivative of barbituric acid.