MCQ – 89362

In a completely and adequately immunized child against diphtheria, a throat swab shows the presence of Corynebacterium diphtheriae-like organisms on Albert staining. These organisms can have which of the following properties on further laboratory processing?

Correct Answer » A Explanation

Clinical Reasoning for the Correct Answer:
– *Corynebacterium diphtheriae* includes both toxigenic and non-toxigenic strains.
– Immunization protects against diphtheria toxin effects, not colonization or carriage by the bacteria.
– All *C. diphtheriae* strains (toxigenic or not) can grow on potassium tellurite medium, which is a selective medium for their isolation.
– Therefore, presence in a vaccinated child’s throat swab means the organism can grow on potassium tellurite, but not necessarily that it is toxigenic.

Why Option A is Correct:
– Growth on potassium tellurite is a basic phenotypic trait of *C. diphtheriae*, regardless of toxigenicity.
– Pink/grey/black colonies on tellurite are characteristic.

Why Option B is Incorrect:
– The Elek’s gel precipitation test detects toxin production (toxigenicity).
– Immunization prevents illness, but does not change the strain’s toxigenicity status.
– Non-toxigenic strains would yield a negative Elek test; immunization does not guarantee the isolated organism is toxigenic.

Why Option C is Incorrect:
– Only toxigenic strains are pathogenic to guinea pigs (in vivo test for diphtheria toxin).
– Presence in an immunized child suggests the possibility of a non-toxigenic strain; simple identification on smear/growth doesn’t confirm pathogenicity.

Why Option D is Incorrect:
– Cytotoxicity in tissue culture is due to toxin production.
– Not all *C. diphtheriae*-like organisms detected post-vaccination produce toxin; immunization status of the child is not relevant to this lab property.

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High‑Yield Synopsis:

Etiology & Pathogenesis
– *Corynebacterium diphtheriae* causes diphtheria via production of an exotoxin.
– Only toxigenic strains cause classical diphtheria disease.

Clinical Features
– Pseudomembranous pharyngitis, lymphadenopathy, possible myocarditis/peripheral neuropathy in toxin-mediated disease.

Differential Diagnoses
– Non-toxigenic *C. diphtheriae*: Isolated from throat but no systemic signs.
– *Corynebacterium pseudodiphtheriticum*: Non-pathogenic; negative on tellurite medium.
– Viral pharyngitis: No membrane, not seen on Albert stain.

Diagnostic Findings
– Albert stain: Shows metachromatic granules.
– Growth on potassium tellurite: Black/grey colonies confirm *C. diphtheriae*.
– Elek test: Demonstrates toxin production (not always positive in all strains).
– PCR for toxin gene (in specialized labs).