**Core Concept**
Flask-shaped ulcers, also known as flask-shaped or oval ulcers, are a characteristic feature of amoebic dysentery caused by Entamoeba histolytica. These ulcers are typically found in the cecum and ascending colon, and their unique shape is due to the amoeba's tendency to erode the mucosa in a specific pattern.

**Why the Correct Answer is Right**
The amoeba's enzymes, particularly proteases and hemolysins, break down the mucosal tissue, leading to the formation of flask-shaped ulcers. The ulcers are often accompanied by a "target lesion" appearance, with a central area of necrosis surrounded by a zone of inflammatory cells. The flask-shaped appearance is due to the amoeba's tendency to erode the mucosa in a circular or oval pattern, often with a narrow neck and a wider base.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because flask-shaped ulcers are not typically associated with shigellosis, which usually presents with small, punched-out ulcers.
**Option B:** This option is incorrect because Crohn's disease can cause a variety of ulcerations, including linear and sergipinous ulcers, but not typically flask-shaped ulcers.
**Option C:** This option is incorrect because tuberculous ulcers are typically larger and more irregular, with a "punched-out" appearance.

**Clinical Pearl / High-Yield Fact**
Flask-shaped ulcers are a classic feature of amoebic dysentery and should prompt a search for Entamoeba histolytica in stool samples or rectal biopsies.

**Correct Answer: C. Tuberculous ulcers.