A fever is an internal response to an external stimulus. A fever is an internal response to an internal stimulus. A fever is an external response to an external stimulus. A fever is an external response to an internal stimulus.
I gave my students this multiple choice question: A fever is a response to a disease-causing agent infecting the body. Raising one's body temperature changes the shape of enzymes in the agent so it cannot reproduce. Which statement best describes a fever?

Only one answer is correct among these 4 choices:
- A fever is an internal response to an external stimulus.
- A fever is an internal response to an internal stimulus.
- A fever is an external response to an external stimulus.
- A fever is an external response to an internal stimulus.

Could you tell me which one is correct?

A fever is an internal response to an internal stimulus.