Which category of parasites typically resides inside the host's body, including the intestinal or organ systems?

• A. Helminths
• B. Ectoparasites
• C. Protozoa
• D. Bacteria

Answer: (A)

Endoparasites live inside the host, occupying internal spaces such as the intestinal tract or other organs. Helminths are the classic internal parasites, being multicellular worms like nematodes, cestodes, and trematodes that spend most or all of their life cycles within the host. They inhabit the gut lumen or migrate into tissues and organs, feeding on host nutrients and sometimes causing disease as they progress through their complex life cycles. Because their defining trait is residing inside the body's internal environments, they fit this description best. In contrast, ectoparasites stay on the outside of the body (skin, hair, clothing), while protozoa and bacteria can also be internal parasites but are typically discussed as microparasites that inhabit cells or tissues rather than as the prominent internal worm category.