National League for Nursing (NLN PAX) Practice Exam

When a hydra senses the presence of dilute nitric acid, it exhibits a biological response known as a stimulus-response behavior. The hydra detects the chemical stimulus (nitric acid), which triggers a reflexive reaction in its body, such as retraction or movement away from the irritant. This response is an example of how organisms respond to environmental stimuli to protect themselves or adapt to changes in their environment.

The stimulus-response mechanism is essential for survival, allowing the hydra to react quickly to potentially harmful substances. This biological response showcases the hydra's ability to perceive changes in its surroundings and initiate appropriate responses to enhance its survival chances.

In this context, while reproduction, movement, and feeding are important biological processes, they are not specifically the immediate reaction to a harmful stimulus like dilute nitric acid. The emphasis here is on how the hydra responds directly to a perceived threat, reinforcing the concept of stimulus-response dynamics in simple organisms.