**Question:** Man is a definitive host for -

A. Entamoeba histolytica
B. Plasmodium vivax
C. Schistosoma mansoni
D. Trypanosoma brucei

**Core Concept:** A definitive host is an organism in a parasite's life cycle where the parasite reproduces sexually. In the context of the question, we are discussing parasites that infect humans and complete their life cycle in humans.

**Why the Correct Answer is Right:**

Man is a definitive host for Schistosoma mansoni because this parasite belongs to the class Trematoda, which is a type of flatworm. Schistosomes, specifically Schistosoma mansoni, require a two-host life cycle:

1. **Incubation Period:** After the cercariae (free-swimming larvae) penetrate the human skin upon contact with the water contaminated with the intermediate host, Biomphalaria glabrata (a snail species). The incubation period for S. mansoni is around 4-6 weeks.
2. **Migration and Establishment:** The cercariae migrate through the body, reaching the mesenteric veins where they penetrate the intestinal mucosa. They mature into schistosomula and then into adult worms in the portal system (venous system) of the liver, spleen, and mesentery.
3. **Reproduction and Egg Production:** The adult worms mate and produce eggs. These eggs are passed out in the stool or urine and reach the environment.
4. **Intermediate Host (snail):** The eggs hatch into miracidia, which penetrate the snail's body. The snail undergoes a series of molts and becomes a sporocyst, then a redia, and finally a cercaria.
5. **The Second Mammalian Host (poor snail-eating mammals):** The cercariae penetrate the skin of aquatic mammals (e.g., pigs, dogs, or humans).

**Why Each Wrong Option is Incorrect:**

A. Entamoeba histolytica (A) is an intestinal protozoan parasite causing amebiasis, which is a separate infection from schistosomiasis.
B. Plasmodium species (B) cause malaria, an infection caused by protozoan parasites of the genus Plasmodium, primarily affecting hepatocytes (liver cells), while schistosomiasis refers to infections caused by Schistosoma species, affecting blood vessels and organs such as liver, spleen, and mesentery.
C. Taenia solium (C) is a parasitic cestode (tapeworm) causing cysticercosis, a condition involving the larval stage of the parasite, not schistosomiasis.
D. Trypanosomes (D) are protozoan parasites causing African sleeping sickness (Human African Trypanosomiasis) or Chagas disease (American Trypanosomiasis), both involving protozoan parasites, but not schistosomes.

**Clinical Sign