Which organism infects humans by larval penetration?

• A. Necator americanus
• B. Enterobius vermicularis
• C. Ascaris lumbricoides
• D. Trichuris trichiura

Answer: (A)

Some human helminths initiate infection when the infective larvae actively penetrate intact skin. Necator americanus, a hookworm, is the classic example of this route. In its life cycle, filariform larvae in soil directly penetrate the skin (often through bare feet), travel via the bloodstream to the lungs, migrate up the bronchial tree to the pharynx, are swallowed, and then reach the small intestine where they attach and feed on blood. This skin-penetration pathway is what sets hookworms apart from the others listed.

Enterobius vermicularis infects by ingesting eggs that are swallowed and hatch in the gut. Ascaris lumbricoides and Trichuris trichiura also establish infection after ingestion of embryonated eggs; their larvae migrate within the body after oral uptake, not via skin penetration.