Which of the following is a sign of infection after orthopedic surgery?

C is the correct answer because the classic signs of infection following orthopedic surgery are characterized by a combination of redness, swelling, warmth, pain, and discharge at the surgical site. These symptoms indicate that the body is responding to an infection, as the immune system activates and blood flow increases to the area, leading to inflammation. The discharge could be purulent, suggesting the presence of bacteria, which is a key indicator of infection.

Other options, while they may indicate different postoperative complications or conditions, do not specifically signify infection. Increased flexibility at a joint could suggest a successful outcome in terms of range of motion but does not relate to infection. Pallor in the skin may indicate poor circulation or compromised healing rather than infection specifically. A reduction in joint mobility is often a result of pain or stiffness but does not directly point to an infectious process. Therefore, the distinctive combination of symptoms in option C is critical for recognizing an infection post-surgery.