**Core Concept**
Pseudo membranous colitis, also known as antibiotic-associated colitis, is a condition characterized by inflammation of the colon and the formation of a pseudomembrane. This condition is often caused by an imbalance in the gut microbiota, leading to the overgrowth of pathogenic bacteria.

**Why the Correct Answer is Right**
The most common organism responsible for pseudo membranous colitis is **Clostridioides difficile (C. difficile)**. This bacterium produces toxins A and B, which cause the inflammation and damage to the colon lining, leading to the formation of a pseudomembrane. C. difficile is often introduced into the gut after the use of broad-spectrum antibiotics, which disrupt the normal balance of gut flora.

**Why Each Wrong Option is Incorrect**
* **Option A:** While Escherichia coli (E. coli) can cause various gastrointestinal infections, it is not the primary cause of pseudo membranous colitis.
* **Option B:** Klebsiella pneumoniae is a bacterium that can cause pneumonia and other infections, but it is not commonly associated with pseudo membranous colitis.
* **Option D:** Pseudomonas aeruginosa is a bacterium that can cause a range of infections, but it is not typically linked to pseudo membranous colitis.

**Clinical Pearl / High-Yield Fact**
When treating patients with pseudo membranous colitis, it is essential to discontinue the use of broad-spectrum antibiotics and consider the use of antibiotics specifically targeted against C. difficile, such as metronidazole or vancomycin.

**Correct Answer: C. Clostridioides difficile.**