**Question:** A 20 year old man presented with hemorrhagic colitis. The stool sample grew Escherichia coli in pure culture. The following serotype of E.coli is likely to be the causative agent -

A. O157
B. O26
C. O111
D. O145

**Correct Answer:** O157

**Core Concept:**

Hemorrhagic colitis is a condition characterized by bleeding in the large intestine, leading to diarrhea and sometimes more serious complications. In this scenario, the patient has E. coli isolated from the stool sample, which indicates an intestinal infection. Escherichia coli (E. coli) is a group of bacteria commonly found in the human gut, but some strains can cause gastrointestinal illnesses.

**Why the Correct Answer is Right:**

Among the given options, Escherichia coli serotype O157 is the most likely causative agent for hemorrhagic colitis. Serotyping is a method used to classify bacteria based on their surface antigens. E. coli O157 belongs to the Shiga toxin-producing E. coli (STEC) group, which is responsible for the production of Shiga-like toxins. These toxins cause damage to the intestinal lining, leading to hemorrhagic colitis.

**Why Each Wrong Option is Incorrect:**

- E. coli O157 is the correct answer, while O26, O111, and O145 are incorrect for the following reasons:

- O26: This serotype is associated with urinary tract infections and is less likely to cause gastrointestinal disorders like hemorrhagic colitis.
- O111: Similar to O26, this serotype is more commonly linked to urinary tract infections rather than gastrointestinal issues.
- O145: While some E. coli strains can cause gastrointestinal infections, O145 is less commonly associated with hemorrhagic colitis compared to O157.

**Clinical Pearl:**

E. coli O157 is a cause of hemorrhagic colitis due to the production of Shiga-like toxins, which damage the intestinal lining, leading to the clinical presentation described. It is crucial for medical students and physicians to recognize and differentiate between different serotypes of E. coli to initiate appropriate treatment and prevent complications.