Nursing Entrance Exam (NEX) Practice Exam

In the context of a hydra reacting to a stimulus, dilute nitric acid serves as a stimulus because it initiates a response from the hydra. When the dilute nitric acid comes into contact with the hydra’s body, it elicits a reaction, typically resulting in behaviors such as contraction or retraction. This is similar to how organisms respond to various environmental changes or substances. In this scenario, the hydra detects the presence of the acid, and the reaction that follows is part of its adaptive mechanism to perceive and respond to external factors.

It is important to differentiate this from other options; a response is the action or change that occurs as a result of the stimulus, an impulse refers to the electrical signal that travels in the nervous system, and a hormone is a chemical messenger that travels through the bloodstream to exert effects on different parts of the body. Therefore, while all these elements are related to how an organism interacts with its environment, the key role of dilute nitric acid in this situation is as the initial stimulus triggering the hydra's reactive behavior.