Which of the following is a sign of infection?

Fever, redness, warmth, and swelling in the affected area are classic signs of infection. When the body detects an infection, the immune system responds by increasing blood flow to the affected area, which causes inflammation. This increased blood flow results in redness and warmth. The fever is a systemic response as the body attempts to create an environment less conducive for the pathogens causing the infection. Swelling occurs due to the accumulation of fluid and white blood cells that are sent to the site of infection to fight off the invading organisms.

The other options reflect different states: weight loss can occur for various reasons, including chronic illness or malnutrition, but is not specific to infection. Increased appetite typically suggests good health or recovery from illness, and improved mobility signifies better physical condition, which again does not indicate infection. Therefore, the presence of fever, redness, warmth, and swelling distinctly indicates an infection, making it the correct answer.