**Core Concept**
Pseudomembranous colitis, also known as Clostridioides difficile (C. difficile) infection, is a type of bacterial infection that causes inflammation of the colon. It is characterized by the formation of pseudomembranes in the colon, which are thick layers of bacteria, white blood cells, and dead cells.

**Why the Correct Answer is Right**
C. difficile is a gram-positive, spore-forming bacterium that produces toxins A and B, which cause the disease. The bacteria are commonly found in the environment, particularly in healthcare settings, and can colonize the gut of individuals who have taken antibiotics, leading to an overgrowth of the bacteria and subsequent infection. The toxins produced by C. difficile damage the lining of the colon, leading to inflammation and the formation of pseudomembranes.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a gram-positive bacterium that can cause a range of infections, including skin infections and pneumonia, but it is not the causative organism of pseudomembranous colitis.

**Option B:** Escherichia coli (E. coli) is a gram-negative bacterium that is commonly found in the gut and can cause urinary tract infections, but it is not the primary cause of pseudomembranous colitis.

**Option D:** Klebsiella pneumoniae is a gram-negative bacterium that can cause pneumonia and other infections, but it is not associated with pseudomembranous colitis.

**Clinical Pearl / High-Yield Fact**
C. difficile infection is often linked to the use of broad-spectrum antibiotics, which can disrupt the normal gut flora and allow the overgrowth of C. difficile. It is essential to consider C. difficile infection in patients who have recently taken antibiotics and present with diarrhea or abdominal pain.

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