**Core Concept**
Mebendazole is a benzimidazole derivative used as an anthelmintic to treat various parasitic worm infections, including hookworm, roundworm, and whipworm. Its mechanism of action involves binding to beta-tubulin in the microtubules of the worm, disrupting microtubule function and ultimately leading to the death of the parasite.

**Why the Correct Answer is Right**
Mebendazole works by inhibiting the polymerization of tubulin into microtubules, which are essential for the motility and division of the worm. This results in the accumulation of the parasite in the intestinal lumen, leading to its eventual expulsion from the body. The mechanism of action of mebendazole is specific to the worm, making it a targeted and effective treatment for parasitic worm infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Mebendazole is not effective against protozoal infections, which is true. Mebendazole is specifically designed to target and kill parasitic worms, not protozoa.
**Option B:** Mebendazole is not metabolized by the liver, which is false. Mebendazole is metabolized by the liver, primarily through the cytochrome P450 system.
**Option C:** Mebendazole has a narrow therapeutic index, which is false. Mebendazole has a relatively wide therapeutic index and is generally well-tolerated.

**Clinical Pearl / High-Yield Fact**
Mebendazole is often used in combination with other anthelmintics to treat complex parasitic infections. It is also essential to note that mebendazole can cause gastrointestinal side effects, such as diarrhea and abdominal pain, in some patients.

**Correct Answer: A. Mebendazole is not effective against protozoal infections.**