**Question:** Drug of choice for pseudomembranous colitis

**Core Concept:** Pseudomembranous colitis (PMC) is a complication of Clostridium difficile (C. difficile) infection causing inflammation and ulceration in the large intestine. The treatment involves suppressing the overgrowth of C. difficile and promoting the growth of beneficial bacteria.

**Why the Correct Answer is Right:** Metronidazole (D) is a nitrogenous antimicrobial agent that inhibits the growth of anaerobic bacteria, including C. difficile. It works by inhibiting the enzyme DNA gyrase and topoisomerase IV, which are essential for bacterial DNA replication, transcription, and repair. By targeting these enzymes, metronidazole effectively reduces the growth of C. difficile and alleviates the symptoms of PMC.

**Why Each Wrong Option is Incorrect:**

A. Vancomycin is an antibiotic that primarily targets gram-positive bacteria, which are not the primary causative agents in C. difficile infection. It does not effectively target C. difficile due to its resistance mechanisms.

B. Nalidixic acid is a quinolone antibiotic that is mainly effective against gram-negative bacteria and is not effective against C. difficile. Moreover, it is rarely used in clinical practice due to its side effects.

C. Tinidazole is a nitroimidazole antibiotic that is effective against both gram-positive and gram-negative bacteria. However, it is less effective than metronidazole for treating C. difficile infection.

**Clinical Pearl / High-Yield Fact:** Metronidazole is the drug of choice for treating pseudomembranous colitis due to its superior efficacy against Clostridium difficile compared to other options provided. In severe cases, combination therapy with vancomycin may be considered. Additionally, understanding the antibiotic resistance mechanisms of C. difficile is crucial for selecting appropriate antibiotics for treatment.