Correct Option: D. Pig Explanation of Why the Answer is Correct: Taenia solium, commonly known as the pork tapeworm, is a parasitic flatworm that primarily infects pigs. The lifecycle of Taenia solium involves two main hosts: the intermediate host (which is typically a pig) and the definitive host (which is usually a human).

1. Lifecycle of Taenia solium:
2. Eggs and Larvae: The lifecycle begins when humans, the definitive hosts, consume undercooked or raw pork containing the larvae (cysticerci) of Taenia solium. Once ingested, the larvae develop into adult tapeworms in the human intestine.
3. Eggs in the Environment: Adult tapeworms produce eggs that are excreted in human feces. These eggs can contaminate the environment, particularly in areas where sanitation is poor.
4. Infection of Pigs: Pigs can become infected by ingesting these eggs from contaminated food, water, or soil. Inside the pig, the eggs hatch and develop into larvae, which then form cysts in the pig's muscles.
5. Human Infection: When humans consume undercooked pork containing these cysts, they can become infected, leading to taeniasis. This can cause various health issues, including abdominal pain and malnutrition.
6. Public Health Implications: The presence of Taenia solium in pigs is a significant public health concern, especially in regions where pork is a dietary staple and where sanitation practices may not be adequate.

Why the Other Options are Wrong or Weaker:

• A. Cow: The tapeworm that primarily infects cattle is Taenia saginata, known as the beef tapeworm. Taenia solium does not have a lifecycle involving cows, making this option incorrect.
• B. Goat: Similar to cows, goats are not hosts for Taenia solium. The tapeworms that may infect goats are different species, and Taenia solium specifically targets pigs.
• C. Dog: Dogs can be hosts for other types of tapeworms, such as Echinococcus species, but they are not involved in the lifecycle of Taenia solium. Dogs do not serve as intermediate or definitive hosts for this particular tapeworm.

Summary of Key Points:

• Taenia solium is the pork tapeworm, primarily found in pigs.
• The lifecycle involves humans as definitive hosts and pigs as intermediate hosts.
• Infection occurs through the consumption of undercooked pork containing cysticerci.
• Other options (cow, goat, dog) do not relate to Taenia solium, as they are associated with different species of tapeworms.

Revision Summary:

• Taenia solium is found in pigs, not in cows, goats, or dogs.
• The lifecycle includes humans and pigs, with humans being the definitive hosts.
• Infection occurs through the consumption of undercooked pork.
• Understanding the lifecycle is crucial for preventing infection and addressing public health concerns.