Which organism is most likely responsible for a 3-year-old with a nonproductive cough, wheezing, and a stool culture showing a 1.6-cm roundworm larva with bilateral pulmonary infiltrates on chest X-ray?

• A. Enterobius vermicularis
• B. Ancylostoma duodenale
• C. Trichuris trichiura
• D. Ascaris lumbricoides

Answer: (D)

Larval migration of a large intestinal nematode through the lungs produces Loeffler-type pneumonia, which explains a nonproductive cough, wheezing, and bilateral pulmonary infiltrates in a young child. Ascaris lumbricoides is classic for this pattern: after ingestion, the larvae travel from the gut to the lungs, causing pulmonary symptoms before returning to the intestine to mature. A stool finding of a relatively large roundworm larva (about 1.6 cm) fits Ascaris, whose larval forms are sizable and distinguish it from other intestinal worms. Enterobius (pinworm) causes perianal itching, Trichuris (whipworm) and hookworms have different presentations and smaller larval forms in stool and don’t typically produce this lung-predominant picture. Therefore, the organism most likely responsible is Ascaris lumbricoides.