MCQ of the Day: 6 November 2026

Traumatic crush injury with foul smell, bronze skin, crepitus, boxcar-shaped Gram-positive bacilli and leucocyte paucity points to clostridial myonecrosis (gas gangrene), most often due to Clostridium perfringens. Its principal virulence factor is alpha toxin, a zinc metallophospholipase C (lecithinase). On egg-yolk agar this lecithinase produces a zone of opalescence (insoluble diglyceride); when half the plate is coated with anti-alpha-toxin antiserum, the opalescence is abolished on that side — the classic Nagler reaction, used for presumptive identification.
A (Terminal round spore / drumstick): morphological hallmark of C. tetani, not C. perfringens, which rarely shows spores in clinical material and does not form terminal spores.
C (Black colonies on bile–aesculin with fishy odour): describes Bacteroides fragilis group; non-sporing Gram-negative anaerobes, not classic traumatic gas gangrene with boxcar Gram-positive rods.
D (Narrow single beta-haemolysis only): C. perfringens typically shows double-zone haemolysis (inner complete from theta toxin/perfringolysin O, outer incomplete from alpha toxin); a narrow single beta zone alone does not support this identification.