A 7-year-old unvaccinated child with sore throat and thick white pseudomembrane; throat culture shows Gram-positive pleomorphic rods; CTBA yields black-gray colonies; Catalase positive; Urea negative. What organism do you suspect and how do you identify it?

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Throat pseudomembrane in an unvaccinated child points to diphtheria caused by Corynebacterium diphtheriae. This organism is a Gram-positive, pleomorphic rod that is catalase positive and urease negative. It grows on cystine-tellurite blood agar with black-gray colonies because it reduces tellurite. To confirm its ability to cause disease, you look for the diphtheria toxin genes; a PCR test for the toxin (A and B) genes is a fast, definitive way to identify toxigenic strains (an Elek test can also be used to demonstrate toxin production). This combination of clinical presentation, characteristic colony morphology on the selective medium, and toxin gene detection makes Corynebacterium diphtheriae the best match. Other options don’t fit the presentation or lab profile: for example, Streptococcus pyogenes would be Gram-positive cocci with beta-hemolysis and would not form the tellurite-reducing, black-gray colonies; Corynebacterium jeikeium is not typically linked to a throat pseudomembrane; Neisseria meningitidis is Gram-negative and presents with different features.