Which of the following is a stimulant?

• A. Caffeine
• B. Valium
• C. LSD
• D. Marijuana

Answer: (A)

Stimulants boost activity in the central nervous system, increasing alertness and energy. Caffeine does this by blocking adenosine receptors in the brain, which normally promote sleepiness. When those receptors are blocked, brain activity rises and neurotransmitters like dopamine and norepinephrine increase, leading to greater wakefulness and a faster heart rate. The other substances don’t act as stimulants: Valium is a depressant that calms brain activity, LSD is a hallucinogen that alters perception, and marijuana has psychoactive effects that aren’t primarily stimulating. So caffeine is the stimulant.