**Core Concept**
The patient's presentation of lower gastrointestinal bleed with sigmoidoscopy showing an ulcer in the sigmoid and biopsy findings of flask-shaped ulcers is consistent with amoebic colitis due to Entamoeba histolytica infection. The flask-shaped ulcers are a characteristic histopathological feature of amoebic colitis.

**Why the Correct Answer is Right**
The correct treatment for amoebic colitis is metronidazole, an antiprotozoal medication that targets the amoeba. Metronidazole works by interfering with the energy metabolism of the amoeba, ultimately leading to its death. The drug is effective against Entamoeba histolytica and is often used as the first-line treatment for amoebic colitis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because vancomycin is an antibiotic used to treat bacterial infections, particularly Clostridioides difficile infection, which is a different condition from amoebic colitis.
* **Option B:** This option is incorrect because corticosteroids are immunosuppressive agents that are not effective against amoebic colitis and may even worsen the infection.
* **Option C:** This option is incorrect because ciprofloxacin is an antibiotic that is used to treat bacterial infections, including gastrointestinal infections, but it is not the primary treatment for amoebic colitis.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between amoebic and non-amoebic colitis, as the treatment approaches differ significantly. The presence of flask-shaped ulcers on biopsy is a key feature that helps in diagnosing amoebic colitis.

**Correct Answer:** C. Metronidazole