On throat swab culture for suspected diphtheria, grey-white pseudomembrane colonies on Loeffler's serum slope show metachromatic granules (volutin) with Albert's stain. To confirm toxin production, the most specific in vitro test is:

• A CAMP test
• B Nagler reaction
• C Quellung reaction
• D Elek's immunodiffusion (gel precipitation) test ✓

Explanation

Elek's test (immunodiffusion in agar) detects diphtheria toxin production: filter paper impregnated with antitoxin is placed in agar; toxin-producing strains form precipitin lines at 45° angles. The CAMP test identifies group B streptococci. The Nagler reaction identifies lecithinase of C. perfringens. The Quellung reaction confirms pneumococcal capsular type.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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