Greetings!
A differential diagnosis is a systematic method of diagnosing disease by noting the symptoms, listening to the patient's description of his complaints, taking the patient's personal and family history, running tests and ultimately arriving at a diagnosis. So, I would assume that to write the essay, you would run through the complaints, symptoms, history, etc. and propose some different diagnoses that fit (the differential part) and then decide which one fits best (the "most likely" diagnosis.
I hope this helps!
Thanks,
Sarah, EssayForum.com
Sarah, EssayForum.com