A wound culture from a road traffic accident victim grows double-zone beta-haemolytic colonies on blood agar. On egg yolk agar, half the plate was pretreated with specific antitoxin before inoculation. No opacity develops over the treated half while the untreated half shows marked opalescence. This observation demonstrates:

• A Lecithinase activity confirming Clostridium botulinum
• B Diphtheria toxin production detected by immunodiffusion lines of identity
• C Alpha toxin (phospholipase C, lecithinase) activity of Clostridium perfringens, inhibited by its antitoxin ✓
• D Hyaluronidase spreading factor of Streptococcus pyogenes

Explanation

This is the Nagler reaction. Alpha lecithinase of Clostridium perfringens splits lecithin in egg yolk medium producing an opaque halo around colonies, and this effect is specifically neutralised on the half plate pretreated with alpha antitoxin. Double-zone haemolysis on blood agar further supports A. perfringens. Botulinum does not cause gas gangrene, and the Elek test described in option B is a gel diffusion method, not egg yolk inhibition.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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