This child is a carrier for C.diphtheriae. Adequate vaccination can prevent the disease but not the colonization.‘Mass immunization might result in disappearance of toxigenic strains from many populations but has no effect on carriage of nontoxigenic strains.’Since the child is completely vaccinated, antitoxin level in serum can always neutralize the toxin but it can not prevent the colonization of nontoxigenic strain. So, the organism can be grown on Potassium tellurite medium but since it might be a nontoxigenic strain, so any test to detect the toxigenicity will be negative.Ref: Apurba Sastry’s Essentials of Medical Microbiology 1/e p246