Which parasite stages are immediately infective for humans after passing through the gastrointestinal tract?

The correct answer relates to the life cycle and infective stages of Cryptosporidium spp. Oocysts are the resistant, environmentally stable forms that can survive outside the host, and they are immediately infective once ingested by a human. When these oocysts are deposited in the gastrointestinal tract, they release sporozoites, which then invade the epithelial cells of the intestines, leading to infection.

In contrast, other choices represent stages or forms that require specific conditions or further development before they can infect humans. Schistosoma spp. eggs must hatch to release miracidia, which then need to find an appropriate host to continue their lifecycle. Toxoplasma gondii bradyzoites, found within tissue cysts, require consumption of tissues containing these cysts to potentially lead to infection, rather than being immediately infective upon gastrointestinal passage. Giardia lamblia trophozoites, although pathogenic, are typically found in the intestines and when passed in feces are in a form (cysts) that needs to be ingested to establish infection in another host. Thus, only the oocysts of Cryptosporidium spp. are capable of immediate infection upon passing through the gastrointestinal tract.