**Core Concept**
The question is testing the student's knowledge of the specific serotype of Escherichia coli that causes hemorrhagic colitis, a severe form of diarrhea characterized by abdominal pain, vomiting, and bloody stools. Hemorrhagic colitis is typically associated with a particular strain of E. coli that produces Shiga toxins.

**Why the Correct Answer is Right**
The correct answer is O 157:H7 because it is the serotype of E. coli that is most commonly associated with hemorrhagic colitis. This strain produces Shiga toxins, which damage the lining of the colon and cause bloody diarrhea. The O 157 antigen is present on the surface of the bacteria, while the H 7 antigen is found on the flagella. The combination of these two antigens makes this strain particularly virulent. The Shiga toxins produced by O 157:H7 bacteria inhibit protein synthesis in the host cells, leading to the characteristic symptoms of hemorrhagic colitis.

**Why Each Wrong Option is Incorrect**
**Option A:** O 159:H7 is not a recognized serotype of E. coli associated with hemorrhagic colitis.
**Option B:** O 107:H7 is not a commonly reported serotype of E. coli in cases of hemorrhagic colitis.
**Option C:** O 55:H7 is not a serotype of E. coli that has been linked to hemorrhagic colitis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that O 157:H7 is a highly virulent strain of E. coli that can cause severe complications, including hemolytic uremic syndrome (HUS), a life-threatening condition characterized by kidney failure and anemia.

✓ Correct Answer: A. O 157:H7