This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
In a completely and adequately immunized child against diphtheria, the throat swab was collected. It showed the presence of C.diphtheriae organisms on Albert staining. These organisms can have one of the following properties on further processing: 
Select and explain. 
A. It can grow on potassium tellurite media, B. It would show +ve Elek’s gel precipitation test, C. It can be pathogenic to experimental guinea pig, D. It can produce cytotoxicity in tissue culture
A: It can grow on potassium tellurite media
Answer: A: It can grow on potassium tellurite media 
Explanation: This child is carrier of diphtheria.

Postassium tellurite is selective media for isolation of diphtheria bacillus from convalescent contact carriers.
Other three test are done for testing virulence, only when isolated strain is C.diphtheria.