Which type of parasite primarily affects the intestines of dogs?

The correct answer is that roundworms primarily affect the intestines of dogs. Roundworms, specifically Toxocara canis, are a common intestinal parasite found in dogs and can cause a range of health issues, especially in puppies. They live in the dog's intestines, where they feed on nutrients from the host and can lead to symptoms such as vomiting, diarrhea, and a bloated abdomen.

In contrast, while tape worms also inhabit the intestines and are a type of intestinal parasite, they usually require an intermediate host (like fleas) to complete their life cycle. Heartworms are a completely different type of parasite that reside in the heart and blood vessels, not the intestines. Fleas are external parasites that live on the skin and fur of animals rather than within the intestines. Understanding the specific locations and impacts of these parasites helps in diagnosing and treating veterinary health issues effectively.