A 4-year-old child with poor sleep and rectal scratching is likely infected with which parasitic agent?

The symptoms presented in the scenario, particularly poor sleep and rectal scratching, are highly indicative of a pinworm infection. Pinworms, which are also known as Enterobius vermicularis, commonly affect children and cause discomfort, especially at night when the female worms migrate to the anus to lay their eggs. This nocturnal activity can lead to significant pruritus (itching), resulting in rectal scratching that can disturb the child's sleep.

Moreover, pinworm infections can spread easily among children, especially in settings like schools and daycare centers, where close contact facilitates transmission. The other parasitic agents listed do not typically present with these specific symptoms or behaviors. Roundworms can cause gastrointestinal symptoms but are less associated with itching. Hookworms mainly lead to anemia or gastrointestinal distress rather than itching. Tapeworms may lead to weight loss or gastrointestinal issues but do not cause rectal itching. Therefore, pinworm infection is consistent with the child's symptoms and is the most likely scenario.