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 causative agent is diagnosed by the - 
Select and explain. 
A. NIH swab method, B. String test, C. Hetrazan test, D. Encysted larvae on muscle biopsy
A: NIH swab method
Answer: A: NIH swab method 
Explanation: The causative agent is Enterobius vermiculais & is diagnosed by NIH swab or perianal swab moistened with saline ie by applying the adhesive surface of cellophane tape to perianal skin in the morning & then this is examined on a glass slide inder microscope. Reference: Harrison20th edition pg 1102