What is the appropriate preposition to use in the sentence: "complaining ___ severe pain"?

Using "complaining of severe pain" is the most appropriate choice because the phrase "to complain of" is commonly used in medical contexts to indicate that a patient is reporting or expressing feelings of discomfort or symptoms. This construction specifically signifies that the individual is experiencing a certain condition—in this case, severe pain.

In medical terminology, "complaining of" effectively conveys the subject presenting their symptoms to a healthcare provider, which helps practitioners understand the patient's condition better. The use of "of" in this context aligns with the formal and precise language typical in health communication.

While other options such as "about" might seem plausible, they do not carry the same connotation in a medical scenario. "With" and "for" are also not conventional with the verb "complain" in this context, as they can imply different relationships that aren't fitting for expressing the experience of pain. Therefore, "complaining of severe pain" is the correct and most contextually appropriate expression.