This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
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 - 
Select and explain. 
A. Filariform larvae, B. Embryonated egg, C. Adult female, D. Adult male
B: Embryonated egg
Answer: B: Embryonated egg 
Explanation: Enterobius vermicularus mainly affects children. The gravid female worm lays ova around the anus ,causing itching especially at night .The ova are often carried to the mouth on the fingers & so re-infection or human to human infection takes place. Reference: Harrison20th edition pg 1120