**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 of the normal gut flora, leading to an overgrowth of toxin-producing bacteria.

**Why the Correct Answer is Right**
The most common cause of pseudo membranous colitis is Clostridioides difficile (C. difficile), a gram-positive, spore-forming bacterium. C. difficile produces toxins A and B, which cause damage to the colon lining and lead to the formation of a pseudomembrane. This bacterium is often associated with the use of broad-spectrum antibiotics, which disrupt the normal balance of the gut flora and allow C. difficile to overgrow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular bacterium or toxin associated with pseudo membranous colitis. While other bacteria may cause similar conditions, C. difficile is the most common cause.

**Option B:** This option is incorrect because it is not a bacterium or toxin associated with pseudo membranous colitis. While certain viruses may cause colitis, C. difficile is the most common cause of this condition.

**Option C:** This option is incorrect because it is not a specific bacterium or toxin associated with pseudo membranous colitis. While certain fungi may cause colitis, C. difficile is the most common cause of this condition.

**Clinical Pearl / High-Yield Fact**
When treating patients with pseudo membranous colitis, it is essential to discontinue the use of broad-spectrum antibiotics and initiate treatment with fidaxomicin or vancomycin to reduce the risk of recurrence.

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