An 8-year-old

girl was brought to the paediatrician showing a bull-neck appearance with a

distinct collar of swelling. The ENT examination revealed a superficial dirty

grey pseudo-membrane covering the tonsils. The microscopic investigation of

this structure showed that this consisted of a fibrino-purulent exudate with

intermingled microorganisms.

Which pathogen

is most likely responsible for these clinico-pathologic findings?