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 -

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