Which compound found in the bark of willow trees is associated with aspirin?

The compound found in the bark of willow trees that is associated with aspirin is salicin. Salicin is a glycoside, which the body can convert into salicylic acid, the active metabolite that is structurally similar to acetylsalicylic acid, commonly known as aspirin. The connections between salicin from willow bark and aspirin are historical and anatomical; salicin was used in traditional medicine for its pain-relieving properties long before aspirin was synthesized.

When willow bark was discovered to alleviate pain, researchers isolated salicin and used it to create aspirin, a more stable and less irritating compound that retains salicin's therapeutic effects. This historical understanding forms the basis for why salicin is commonly associated with aspirin.

In contrast, berberine, caffeine, and epinephrine do not share this direct relationship with aspirin. Berberine is known for its antimicrobial and anti-inflammatory properties, caffeine is a stimulant found in coffee and tea, and epinephrine is a hormone that plays a critical role in the body's fight-or-flight response. Each of these compounds serves distinct functions that do not relate to the analgesic effects of aspirin.