**Core Concept**
The question tests the student's knowledge of the specific serotypes of **Escherichia coli** that are associated with **hemorrhagic colitis**. Hemorrhagic colitis is a type of foodborne illness characterized by bloody diarrhea, and certain strains of E. coli are known to cause this condition. The **Shiga toxin** produced by these bacteria is responsible for the severe symptoms.

**Why the Correct Answer is Right**
The correct answer, **0 157 : H7**, is a specific serotype of E. coli known to produce **Shiga toxin**, which can cause severe **hemorrhagic colitis**. This serotype is commonly associated with outbreaks of foodborne illness, particularly those linked to undercooked ground beef, unpasteurized apple cider, and contaminated produce. The **Shiga toxin** inhibits protein synthesis in the intestinal epithelial cells, leading to cell death and the characteristic bloody diarrhea.

**Why Each Wrong Option is Incorrect**
**Option B:** **0 159 : H7** is not a recognized serotype associated with hemorrhagic colitis.
**Option C:** **0 107 : H7** is also not a known serotype linked to this condition.
**Option D:** **0 55** is a serotype of E. coli, but it is more commonly associated with **urinary tract infections** rather than hemorrhagic colitis.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **E. coli 0 157 : H7** is a significant cause of foodborne illness, and its diagnosis is often based on stool culture and detection of **Shiga toxin**. Prompt identification and treatment are essential to prevent complications, such as **hemolytic uremic syndrome**.

**✓ Correct Answer: A. 0 157 : H7**