A throat swab from a suspected diphtheria case is inoculated on Loeffler's serum slope. After 12 hours, smears stained with Albert's stain are examined. The finding that supports Corynebacterium diphtheriae is:

• A Black colonies with a brown halo around them
• B Acid-fast beaded bacilli on Ziehl-Neelsen stain
• C Metachromatic granules staining bluish-black against a green cytoplasm ✓
• D Double zone of beta-haemolysis on blood agar

Explanation

Corynebacteria store polymerised metaphosphate as volutin granules, which take up the bluish-black colour with Albert's methylene blue while the cytoplasm stains green, showing the classical metachromatic granulation. Black colonies with a brown halo describe growth on potassium tellurite agar, a different medium used later in the workup. Acid-fastness suggests mycobacteria, and double-zone haemolysis is typical of Clostridium perfringens on blood agar.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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