The slender rhabditiform larvae of which of the following helminths move about in water and are ingested by species of cyclops?
**Core Concept**
The question tests the student's knowledge of the life cycle of certain helminths, specifically the larval stages that involve water and a specific intermediate host, the cyclops. This is a critical concept in parasitology, as it determines the mode of transmission of these parasites.
**Why the Correct Answer is Right**
The correct answer is **D. Dracunculus medinensis**. The slender rhabditiform larvae of Dracunculus medinensis, also known as the Guinea worm, are free-living in water and are ingested by species of cyclops, a type of small crustacean. The larvae then undergo further development within the cyclops, eventually encysting in the muscle tissue of the host. When an infected cyclops is ingested by a human, the larvae are released and begin to migrate through the body, eventually emerging through the skin.
**Why Each Wrong Option is Incorrect**
* **Option A:** Ascaris lumbricoides is a large roundworm that is ingested by humans through contaminated food or water, but it does not have a larval stage that involves cyclops as an intermediate host.
* **Option B:** Trichuris trichiura, also known as the whipworm, is ingested by humans through contaminated food or water, but it does not have a larval stage that involves cyclops as an intermediate host.
* **Option C:** Hookworms, such as Ancylostoma duodenale and Necator americanus, are ingested by humans through contaminated soil, but they do not have a larval stage that involves cyclops as an intermediate host.
**Clinical Pearl / High-Yield Fact**
The life cycle of Dracunculus medinensis involves a complex series of intermediate hosts, including cyclops, and is unique among helminths. This makes it a critical concept to understand in the diagnosis and treatment of dracunculiasis, a disease that can cause significant morbidity and mortality in affected populations.
**Correct Answer: D. Dracunculus medinensis**