**Core Concept:** Schistosomiasis is a parasitic infection caused by flatworms of the genus Schistosoma. The disease is transmitted to humans through water contaminated with cercarial larvae, which are released by infected freshwater snails. The clinical manifestations involve an immune response to the worms and their eggs.

**Why the Correct Answer is Right:** Schistosoma species primarily infect humans at the larval (schistosome) stage. The life cycle of these parasites involves the intermediate host, typically freshwater snails, which become infected by consuming infected freshwater bivalve mollusks. The snails then release cercarial larvae into the water, which can penetrate human skin when coming into contact with contaminated water.

**Why Each Wrong Option is Incorrect:**

A. Schistosomiasis occurs at the adult stage (mature worms). This answer is incorrect because adult schistosomes do not directly infect humans.

B. This answer is incorrect because schistosome eggs, not adult worms, are responsible for causing the disease in humans.

C. This answer is incorrect as the primary infection occurs through cercarial larvae, not the intermediate host stage.

D. This answer is incorrect because the infection primarily occurs through the penetration of cercarial larvae into human skin, not the consumption of contaminated water.

**Clinical Pearl:** The correct understanding of schistosomiasis life cycle is essential for diagnosing and treating the disease. Schistosomiasis can be diagnosed through serological tests, microscopy, and molecular techniques. Treatments include praziquantel, which is effective against adult worms but not eggs. Prevention involves avoiding contact with contaminated water and proper sanitation.