A pharyngeal specimen is inoculated on cystine-tellurite blood agar, modified Tinsdale agar, and MacConkey agar. After 48 hours of incubation, black colonies with a brown halo are observed on the Tinsdale agar and gray colonies are isolated from the cystine-tellurite agar. However, no growth is observed on the MacConkey agar. Which one of the following statements explains this discrepancy?
a. The MacConkey agar is most likely ex-pired.
b. The MacConkey agar should be incubated an additional 24 hours to observe growth.
c. The pH of the media may be altered, causing an environment that is too acidic for the organism to grow.
d. No discrepancy is present; this organism does not grow on MacConkey agar.
D
The organism is C. diphtheria and therefore can grow on Tinsdale agar but fails to grow on MacConkey agar.
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