What is indicated by the term pyrexia?

The term "pyrexia" refers specifically to a rise in body temperature, which is commonly known as fever. It is often a response to infection, inflammation, or other medical conditions. When the body detects pathogens, the immune response may lead to an increase in temperature as a way to create an environment that is less favorable for those pathogens. This elevation in body temperature is a critical part of the body's defense mechanisms.

In contrast, the other terms listed in the options refer to different medical concepts entirely. A decrease in blood pressure relates to hypotension, a condition that can arise from various medical issues. Severe pain is typically not termed pyrexia but may relate to conditions causing distress or injury. An allergic reaction involves an immune response to allergens, which does not inherently include changes in body temperature as a primary indicator. Therefore, the definition and implications of pyrexia directly correspond to a rise in the body's temperature.