Following acute pharyngitis, a patient was on broad spectrum antibiotics. One week later he developed watery diarrhoea with a foul odour and abdominal cramps. Antibiotic induced colitis is confirmed. What is the most common cause of antibiotic induced colitis?

**Core Concept**
Antibiotic-induced colitis, also known as antibiotic-associated diarrhea (AAD), is a condition where the use of broad-spectrum antibiotics disrupts the normal balance of gut flora, leading to an overgrowth of pathogenic bacteria. This disruption can cause symptoms such as watery diarrhea, abdominal cramps, and foul odor.

**Why the Correct Answer is Right**
The most common cause of antibiotic-induced colitis is the overgrowth of Clostridioides difficile (C. difficile), a Gram-positive, spore-forming bacterium. C. difficile produces toxins A and B, which cause inflammation and damage to the colonic epithelium, leading to the characteristic symptoms of antibiotic-induced colitis. The use of broad-spectrum antibiotics such as clindamycin and cephalosporins can disrupt the normal balance of gut flora, allowing C. difficile to overgrow and produce toxins.

**Why Each Wrong Option is Incorrect**
**Option A:** While Escherichia coli (E. coli) can cause diarrhea, it is not the most common cause of antibiotic-induced colitis.
**Option B:** Staphylococcus aureus can cause food poisoning, but it is not typically associated with antibiotic-induced colitis.
**Option C:** Klebsiella pneumoniae can cause urinary tract infections and pneumonia, but it is not the most common cause of antibiotic-induced colitis.

**Clinical Pearl / High-Yield Fact**
C. difficile infection is often associated with recent antibiotic use, particularly clindamycin and cephalosporins, and can be diagnosed using a stool toxin assay or PCR.

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

✓ Correct Answer: B. Clostridium difficile