**Core Concept**
The question is testing knowledge of intestinal parasitic infections, specifically a common condition causing nocturnal perianal pruritus in school-age children worldwide. Enterobius vermicularis, also known as the pinworm, is a nematode worm that infects the human intestine.

**Why the Correct Answer is Right**
Enterobius vermicularis is the most common intestinal parasite worldwide, particularly in school-age children. The female worm lays eggs around the anus, causing intense itching (pruritus) at night, which leads to scratching and potentially spreading the eggs to other family members. The eggs hatch into larvae, which then penetrate the intestinal wall and mature into adult worms. **The presence of adult worms in the stool is a key diagnostic clue for Enterobius infection.**

**Why Each Wrong Option is Incorrect**
**Option A:** Plasmodium vivax is a protozoan parasite that causes malaria, a different type of infection altogether. While it can cause significant morbidity, it is not typically associated with nocturnal perianal pruritus or the presence of adult worms in stool.

**Option C:** Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery, characterized by bloody diarrhea and abdominal pain. It is not commonly associated with pruritus or the presence of adult worms in stool.

**Option D:** Strongyloides stercoralis is a nematode worm that causes strongyloidiasis, a condition characterized by gastrointestinal symptoms and skin lesions. While it can cause pruritus, it is not typically associated with the presence of adult worms in stool.

**Clinical Pearl / High-Yield Fact**
The most effective treatment for Enterobius vermicularis is a single dose of mebendazole or albendazole, which should be administered to all family members to prevent re-infection.

**✓ Correct Answer: B. Enterobius vermicularis**