A molecule that enters the body from an external source and alters its normal functions is a(an)

Question

Quizplus · Accepted Answer

The definition given matches the definition of a drug. A drug is any substance that enters the body and alters its normal functions, usually with a therapeutic or diagnostic purpose.

Option A: An agonist is a molecule that binds to and activates a receptor to produce a biological response. It is not necessary for it to come from an external source. 
Option B: An antagonist is a molecule that binds to a receptor but does not activate it. It blocks the receptor from being activated by other molecules. Again, it is not necessary for it to come from an external source. 
Option D: Hormones are chemical messengers produced by the body's endocrine glands that travel through the bloodstream to target cells. They regulate many bodily functions, but they are not necessarily from external sources. 
Option E: Steroids are a class of organic molecules with a characteristic structure, but not all steroids are drugs that alter bodily functions.

A Molecule That Enters the Body from an External Source