**Core Concept**
The patient's presentation of lower gastrointestinal bleed with sigmoidoscopy findings of ulcers in the sigmoid colon and biopsy showing flask-shaped ulcers is suggestive of amoebic colitis, a condition caused by the protozoan parasite *Entamoeba histolytica*. This condition is characterized by the invasion of the intestinal mucosa by the parasite, leading to the formation of flask-shaped ulcers.

**Why the Correct Answer is Right**
The correct treatment for amoebic colitis is metronidazole, an antibiotic that targets the parasite. Metronidazole works by inhibiting the synthesis of DNA in the parasite, ultimately leading to its death. The mechanism of action of metronidazole involves the formation of free radicals that damage the parasite's DNA, ultimately leading to cell death. This is a result of the reduction of metronidazole to its active form by the enzyme flavoprotein in the parasite.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the underlying cause of the patient's condition. While antacids may help to neutralize stomach acid and provide temporary relief from symptoms, they do not treat the underlying infection caused by *Entamoeba histolytica*.
* **Option B:** This option is incorrect because it is a treatment for bacterial infections, not amoebic colitis. Antibiotics such as ciprofloxacin target bacterial infections, not protozoan parasites like *Entamoeba histolytica*.
* **Option D:** This option is incorrect because it is a treatment for inflammatory bowel disease (IBD), not amoebic colitis. Corticosteroids such as prednisolone are used to reduce inflammation in conditions like Crohn's disease and ulcerative colitis, but they are not effective against amoebic colitis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that *Entamoeba histolytica* can cause both invasive and non-invasive forms of amoebiasis. Invasive amoebiasis can lead to severe complications, including amoebic liver abscess and intestinal perforation, while non-invasive amoebiasis may present with asymptomatic carriage or mild gastrointestinal symptoms.

**Correct Answer:** C. Metronidazole