## Core Concept
The lifecycle of certain parasites involves two hosts: a definitive host where the parasite reaches maturity and reproduces sexually, and a secondary or intermediate host where asexual reproduction occurs or the parasite develops without reproducing. The question pertains to identifying a parasite for which humans serve as a secondary host.

## Why the Correct Answer is Right
The correct answer, *Taenia solium*, is a tapeworm for which humans act as an intermediate or secondary host when they ingest eggs from the environment contaminated with feces from an infected individual. In humans, the larvae develop but do not mature into adult worms; this occurs in pigs, the definitive host. Humans can become infected through fecal-oral contamination or through autoinfection from an existing infection.

## Why Each Wrong Option is Incorrect
* **Option A:** *Taenia saginata* - Humans are the definitive host for *T. saginata*, not the secondary host. The larvae develop in cattle.
* **Option B:** *Echinococcus granulosus* - While humans can serve as an intermediate host for *E. granulosus*, the lifecycle primarily involves canines as the definitive host. Humans are not typically described as secondary hosts in the traditional sense for this parasite; they are aberrant intermediate hosts.
* **Option D:** *Ancylostoma duodenale* - This hookworm primarily infects humans directly through the skin or orally. Humans are the primary host; there isn't a typical secondary host in its lifecycle.

## Clinical Pearl / High-Yield Fact
A key point to remember is that *Taenia solium* can cause cysticercosis in humans, a serious condition where the larvae develop in tissues, including the brain (neurocysticercosis). This condition highlights the importance of proper sanitation and hygiene to prevent *T. solium* infections.

## Correct Answer Line
**Correct Answer: C. Taenia solium**