A patient has a fever three days post-op, WBC count of 15,000, negative blood cultures, and eosinophils at 9%. What is the likely diagnosis?

In this scenario, the patient's presentation is indicative of a drug fever. The patient has a fever occurring three days post-operation, which is a time frame where reactions to medications are known to manifest, especially if new medications have been introduced in the perioperative period.

The elevated WBC count of 15,000 suggests a possible inflammatory or immune response; however, it is not exclusively indicative of a bacterial infection, especially since blood cultures are negative. The presence of eosinophils at 9% is particularly telling; elevated eosinophils can be associated with drug reactions, allergic responses, and some forms of infection, but in the context of recent surgical intervention and new medications, this elevation aligns more closely with a drug-induced fever.

Other options, such as viral or bacterial infections, would typically produce positive findings in blood tests or cultures after this period if they were the cause of the fever. Malignant hyperthermia, while a serious concern post-anesthesia, usually presents much more acutely and with distinct symptoms, including severe muscle rigidity and metabolic derangements, which are not described in this case. The clinical picture strongly points to a drug fever due to the timeline and laboratory findings presented.