Which of the following is a sign of a strain?

A strain is an injury to a muscle or tendon that typically occurs when the muscle fibers are stretched or torn. The primary indicator of a strain is sudden pain, which often occurs at the moment of injury. This pain can range from mild to severe, depending on the extent of the injury.

In the context of the options provided, sudden pain signifies the immediate response of the body to the injury. It indicates that there has been some level of damage to the muscle or tendon.

The other options, such as high fever, loss of consciousness, and hives, are not typical signs of a strain. High fever could indicate an infection or another underlying health issue, loss of consciousness might relate to a more serious condition such as a head injury or shock, and hives are usually a reaction to allergens rather than a physical strain on muscles or tendons. Thus, the identification of sudden pain as a sign of a strain highlights the acute nature of this injury and its immediate symptoms.