**Question:** Most common drug causing pseudomembranous enterocolitis is

A. Ciprofloxacin
B. Metronidazole
C. Amoxicillin
D. Tetracycline

**Correct Answer:** B. Metronidazole

**Core Concept:** Pseudomembranous enterocolitis, also known as Clostridium difficile-associated disease (CDAD), is an inflammatory condition of the colon that occurs as a result of antibiotic use. Clostridium difficile is a bacterium that thrives in an antibiotic-depleted gut environment, leading to toxin production and inflammation, which causes symptoms such as diarrhea, abdominal pain, and sometimes severe complications like toxic megacolon or pseudomembranes formation.

**Why the Correct Answer is Right:** Metronidazole is a nitroimidazole antibiotic commonly used to treat anaerobic infections. However, it has a high propensity to disrupt the normal gut flora, allowing Clostridium difficile to overgrow and produce toxins. These toxins lead to the damage of the colonic mucosa and eventually cause pseudomembranous enterocolitis.

**Why Each Wrong Option is Incorrect:**

A. Ciprofloxacin (A) is a fluoroquinolone antibiotic that can disrupt gut flora, but its incidence of causing CDAD is generally lower than metronidazole.

B. Tetracycline (D) is a broad-spectrum antibiotic that can disrupt gut flora and lead to CDAD, but its incidence is generally lower than metronidazole.

C. Amoxicillin (C) is a penicillin antibiotic primarily targeting Gram-positive bacteria, and its incidence of causing CDAD is generally lower than metronidazole.

**Clinical Pearl:**

When prescribing antibiotics, clinicians should be mindful of the potential disruption of the normal gut flora and the risk of CDAD. Metronidazole and tetracycline are more likely to cause CDAD compared to amoxicillin and ciprofloxacin. Monitoring patients on antibiotics for signs and symptoms of CDAD, such as diarrhea, abdominal pain, and fever, is crucial for early detection and management.