A 6-year old child who presented with perianal pruritus, excoriation of skin and nocturnal enuresis was found to be infected with a parasite causing autoinfection. The infective form of parasite for humans is the –
**Core Concept**
The question is testing the student's knowledge of the life cycle and transmission of a specific parasite that causes autoinfection in humans. The key concept here is the identification of the infective form of the parasite, which is essential for understanding the mode of transmission and prevention of the infection.
**Why the Correct Answer is Right**
The correct answer is the embryonated egg, which is the infective form of the parasite *Enterobius vermicularis*, also known as the pinworm. The parasite causes autoinfection through the ingestion of embryonated eggs, which are usually ingested while contaminated fingers are placed in the mouth. The eggs hatch in the small intestine, and the larvae mature into adult worms. The adult female worms lay eggs on the perianal skin, which can cause perianal pruritus and excoriation of the skin. The nocturnal enuresis may be due to the intense perianal itching, which can cause the child to wake up during the night.
**Why Each Wrong Option is Incorrect**
**Option A:** Filariform larvae are the infective form of certain nematodes, such as *Strongyloides stercoralis*, but not of *Enterobius vermicularis*.
**Option C:** Adult female worms are present in the human intestine, but they do not cause autoinfection. Autoinfection occurs through the ingestion of embryonated eggs.
**Option D:** Adult male worms are also present in the human intestine, but they do not cause autoinfection. The male worms do not lay eggs.
**Clinical Pearl / High-Yield Fact**
It is essential to note that the diagnosis of pinworm infection can be confirmed by the Scotch tape test, where a piece of Scotch tape is pressed onto the perianal skin in the morning to collect eggs. This test is a simple and non-invasive method for diagnosing pinworm infection.
**✓ Correct Answer: B. Embryonated egg**