In which situation is modifier 24 appropriately used?

Modifier 24 is specifically used to indicate an unrelated evaluation and management (E/M) service by the same physician during the postoperative period. This modifier is applied when a physician provides care for a different issue that is not associated with the surgical procedure that was performed and falls within the global surgical period.

In this context, option C is correct because it pertains to a scenario where a patient presents a new and unrelated condition or problem that requires evaluation and management by the same physician who performed the surgery. The use of modifier 24 clarifies that the physician's services are for a different diagnosis rather than a complication or normal postoperative care associated with the surgical procedure.

The other options do not correctly apply to modifier 24 as follows: additional testing related to a surgical procedure would typically be billed with a different modifier rather than 24. Anesthesia for a surgical procedure does not use modifier 24 and is billed separately based on the anesthesia codes. Complications arising from a surgery are generally addressed within the global period of the surgical service rather than being labeled with modifier 24.