What term refers to anything the body is able to detect by means of its receptors?

• A. Stimulus
• B. Reflex
• C. Signal
• D. Response

Answer: (A)

The key idea is that a stimulus is anything that can be detected by the body's receptors. Sensory receptors in the skin, eyes, ears, nose, and tongue are tuned to pick up changes in the environment—like light, sound, heat, pressure, and chemicals—and convert that energy into nerve signals that the brain can interpret. That triggering event is what we call a stimulus.

Why this is the best choice: a stimulus is specifically the detectable event or change that receptors respond to. The other terms describe what happens after detection or the type of reaction, not the trigger itself. A signal is the information carried after detection (the neural message). A reflex is a quick, automatic response to a stimulus. A response is the action produced by the body after processing the stimulus.