An Egg of the Organism of S

An egg of the organism of S.mansoni is isolated from the feces of an adult man who has been traveling in Africa.An unoperculated egg with a large lateral spine is recovered.The patient is administered the standard dose of praziquantel;however,his infection does not improve.Which one of the following statements explains this discrepancy?

A) Metrifonate should have been administered.
B) Bilarcil is the drug of choice for this type of infection.
C) Steroids should have been initially administered.
D) A larger dose may be required to be effective against this organism.