A 17-year-old high school student presents with persistent sore throat and malaise not relieved by penicillin therapy. What would you order next?

In this scenario, where a 17-year-old student exhibits persistent sore throat and malaise that does not respond to penicillin treatment, ordering a Monospot test is the most appropriate next step. This test is specifically designed to detect heterophile antibodies, which are often present in infectious mononucleosis caused by Epstein-Barr virus (EBV).

Patients with mononucleosis may present with symptoms similar to those of strep throat, including a sore throat and malaise, but they often do not improve with antibiotics such as penicillin, which is effective against bacterial infections. The Monospot test helps identify this viral infection, allowing for appropriate management and treatment.

A throat culture, while useful for detecting streptococcal infections, is less relevant in this case because the patient has already received penicillin, which would not help if the cause of the sore throat is viral. A rapid antigen test can identify streptococcal infections quickly but would also not address the likely viral etiology indicated by the patient's symptoms. The Thayer-Martin plate test is primarily used for isolating Neisseria gonorrhoeae and is not appropriate for this clinical presentation. Thus, the Monospot test is the best choice