Which term describes a sudden spike in fever?

The term that best describes a sudden spike in fever is "acute fever." Acute fever refers to a rapid increase in body temperature that often occurs in response to an infection or illness. This term indicates that the fever appears suddenly and is usually of a short duration, aligning with the clinical presentation of many acute illnesses.

In contrast, cyclic fever refers to a fever that resolves and then reoccurs at regular intervals; febrile describes the state of having a fever but does not specify the nature of its onset, and continuous fever is characterized by a sustained elevation in temperature without significant fluctuation. Therefore, "acute fever" is the most precise term that captures the essence of a sudden spike in fever, making it the correct choice.