Which reaction is not an initial response of the body to injury?

Atrophy is not an initial response to injury because it refers to a gradual process of tissue degeneration or decrease in size due to disuse or a lack of stimulation over time. In contrast, swelling, pain, and redness are immediate physiological responses that occur shortly after an injury.

Swelling, or edema, happens as a result of increased blood flow and the accumulation of fluid at the injury site, which is part of the inflammatory response. Pain typically arises from irritation of nerve endings, which can be caused by damage to tissues, inflammation, or chemical mediators released during the injury process. Redness is due to the dilation of blood vessels, allowing more blood to reach the damaged area, contributing to the warmth and visible inflammation. Thus, while swelling, pain, and redness are acute responses meant to protect and heal the injury, atrophy develops later if the injury leads to inactivity or a lack of use of the affected body part.