A child is admitted with bull-neck, hoarseness, low-grade fever and a grey-green pseudomembrane extending from the tonsil to the soft palate. Loeffler's serum slope culture shows club-shaped bacilli in 'Chinese letter' arrangement. Which laboratory test confirms the toxigenicity of the isolated organism?
- A Tellurite reduction test
- B Metachromatic granule demonstration by Albert's stain
- C Virulence testing on guinea pig skin (intradermal inoculation)
- D Elek's gel precipitation (immunodiffusion) test ✓
Explanation
Elek's immunodiffusion test is the standard in-vitro test for diphtheria toxin production; antitoxin-impregnated filter paper is laid perpendicular to test and control culture streaks on a special agar plate, and precipitation bands at 45° confirm toxigenicity. Albert's stain with metachromatic (volutin) granules confirms the morphology of C. diphtheriae but does not assess toxigenicity. Tellurite reduction turns colonies grey/black on selective media, aiding isolation. Guinea pig inoculation is the in-vivo reference but is rarely used now that PCR for tox gene is available.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.