A 6-year-old unvaccinated child from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever, and difficulty swallowing. On examination, there is a thick, adherent, greyish-white membrane over the tonsils and pharynx that bleeds on attempted removal. The child also has cervical lymphadenopathy and mild stridor. Which investigation is most appropriate to confirm the diagnosis of diphtheria?