A 24-year-old man presents with two days of sore throat, fever and increasing difficulty swallowing. He has muffled speech, tender cervical nodes and a grey-white layer over both tonsils; his breathing is currently unlaboured and his oxygen saturation is 98% on room air. The layer leaves a bleeding surface when he tries to remove it, and he returned from a region with low childhood immunisation coverage one week ago. Which one of the following is the most appropriate immediate treatment?