A 20-year-old man is admitted to the Emergency Department with a severe sore throat. He
arrived a few days earlier from Central America to visit a relative. He is extremely lethargic,
with a fever, and is finding it increasingly difficult to swallow. His temperature is 38.1 C, his
blood pressure is 105/80 mmHg, and his pulse is 72 beats per minute (bpm) and regular.
Investigation reveals a grey pseudomembrane forming on the posterior pharynx.

Which is the most important initial investigation for this patient?