A 3-year-old child develops sudden onset fever, sore throat, and a greyish-white membrane firmly adherent to the tonsillar surface extending to the uvula. Attempting to remove the membrane causes bleeding. Gram stain shows club-shaped organisms in Chinese letter arrangement. Which test confirms the toxigenicity of the isolate?

• A CAMP test on blood agar
• B Nagler's reaction on egg yolk agar
• C Optochin susceptibility test
• D Elek's gel precipitation (immunodiffusion) test ✓

Explanation

Corynebacterium diphtheriae toxigenicity is confirmed by the Elek's immunodiffusion test in which the isolate is streaked perpendicular to a filter paper strip impregnated with diphtheria antitoxin; if toxin is produced, precipitin lines form at an angle where toxin and antitoxin meet. The diphtheria toxin is encoded by the tox gene carried by bacteriophage beta; only lysogenised strains produce toxin. CAMP test confirms Streptococcus agalactiae; Nagler's reaction detects Clostridium perfringens lecithinase; optochin differentiates Streptococcus pneumoniae.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.