Which bacterium is commonly associated with enterocolitis (dysentery) and is a non-lactose fermenter?

• A. Escherichia coli
• B. Salmonella typhi
• C. Shigella dysenteriae
• D. Campylobacter jejuni

Answer: (C)

The key idea is that bacillary dysentery points to an invasive enteric pathogen that does not ferment lactose. Shigella dysenteriae fits this perfectly: it invades the colonic mucosa, causing inflammation, ulcers, and the bloody, mucus-rich stools of dysentery. On lab plates like MacConkey, it fails to ferment lactose, so its colonies stay colorless, a hallmark of non-lactose–fermenting organisms. This combination—the clinical picture of dysentery plus the non-lactose fermentation trait—makes Shigella dysenteriae the best answer. Other organisms may cause diarrhea or be non-lactose fermenters, but they don’t align as cleanly with both the invasive, dysenteric presentation and lactose-negativity.