**Core Concept**
The patient's presentation of lower gastrointestinal bleeding with sigmoidoscopy findings of ulcers in the sigmoid colon, along with a biopsy showing flask-shaped ulcers, is suggestive of amoebic colitis caused by Entamoeba histolytica. The flask-shaped ulcers are a hallmark of this condition.

**Why the Correct Answer is Right**
The correct answer is based on the understanding that amoebic colitis is typically treated with metronidazole, an anti-amoebic agent that targets the trophozoites of Entamoeba histolytica. Metronidazole works by inhibiting the pyruvate ferredoxin oxidoreductase enzyme, which is essential for the energy metabolism of the amoeba. This leads to the death of the amoeba, thereby resolving the infection and its associated symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ciprofloxacin is an antibiotic that targets bacterial infections, not protozoal infections like amoebic colitis.
**Option B:** This option is incorrect because mesalamine is used to treat inflammatory bowel disease (IBD), not amoebic colitis. Mesalamine works by reducing inflammation in the gut.
**Option C:** This option is incorrect because sulfasalazine is also used to treat IBD, not amoebic colitis. Sulfasalazine works by reducing inflammation in the gut.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the diagnosis of amoebic colitis can be challenging, and a high index of suspicion is necessary. The presence of flask-shaped ulcers on biopsy is a key diagnostic feature of this condition.

**Correct Answer:** C.