MCQ of the Day: 23 October 2026

Answer: B. Elek gel immunodiffusion test. The picture is faucial diphtheria caused by Corynebacterium diphtheriae, which has re-emerged in India among under-immunised children and adolescents (for example, Kerala in 2016). Not every isolate is toxigenic: the toxin is encoded by the tox gene carried on a lysogenic beta-corynephage. Toxigenicity is shown in vitro by the Elek test, in which a filter-paper strip soaked in antitoxin is laid across a serum agar plate and lines of precipitation form where toxin from the streaked isolate meets antitoxin. PCR for the tox gene is a quicker screen, but it detects the gene rather than expressed toxin, so Elek remains the confirmatory test.
A. The Schick test is an intradermal test of the patient's immunity to diphtheria toxin, not of the isolate's toxigenicity, and is now obsolete.
C. The Nagler reaction detects the alpha-toxin (lecithinase) of Clostridium perfringens on egg-yolk agar.
D. The CAMP test identifies Streptococcus agalactiae (and is reversed by C. pseudotuberculosis phospholipase D), not diphtheria toxin.